Secondhand is preferable - though always personally inspect the device for any unacceptable defects.
Safest bet, as OnePlus devices receive the most developer support. However, if you care about the headphone jack, do not pick 6T & above. In addition, avoid their 7T & later releases if you need to use custom ROMs for now.
Personal warning : haven't experienced any OnePlus PDAs yet, so take with a grain of salt.
Anti-features : no microSD for ALL OnePlus PDAs, so OTG for every flashing.
Personal warning : I only experienced 1 device - Zenfone 6.
Another safe bet, and one I would call a close 2nd, edging Google off due to hardware advantages such as microSD availability. OnePlus still has the sheer advantage of active developments, unfortunately.
Anti-features : no bootloader re-locking (officially).
Third safest bet (Used to be 2nd, but Asus took it), and the most ironic one. However, as Google isn't the best with hardware, there's bound to be issues.
Anti-features : no microSD for ALL models & single SIM only.
For the filthy casuals who don't like Android. Full stop. Their performance blows all Androids off - that's all I shall admit.
Despite the issues, the V30 is 2017's best underrated PDA, and holds a strong point even in 2019. The regular V30 only had 64GB storage, & V30S has availability issues (though, if you get the V30s, you get 2GB extra RAM).
Pros:
Lifestyle improvement. However, input-only.
Fast enough to run PUBG at Smooth/Extreme. However, as a drawback of doing this, the device will get hot.
It's not just present, its performance outstrips most.
Really black blacks, or, in other words, Turned-off black.
For this size, the weight doesn't feel premium. Though, it'll save the pants from dropping due to cargo - another lifestyle improvement.
Something I don't recommend doing - but it really works. Sidesteps the "visible front camera" issue the F1 has.
Cons:
To quote JerryRigEverything, "glass is glass, and glass breaks". Despite the MIL-STD-810G certification, it's still a glass sandwich. Also, glass sandwich build means that repairing this PDA requires removing the glued-on back glass...with a heat gun.
Replacing the screen requires either destroying the old one as it's being removed from the metal body (for glass only), or replace both altogether.
Takes a while getting used to. And yeah, my muscle memory got kinda messed up - to the point of occasionally confusing the F1's fingerprint scanner as the power button.
Requires plugging the PDA to a PC, then pressing down on the volume down & power button until the 1st sign of display turning on, immediately releasing & re-pressing the power button afterwards. The other 3 PDAs I've experienced merely required holding down volume down & power button to get there - no plugging needed (unless for adb / fastboot operations).
As stated by the fastboot issues above, volume up + power does not boot to recovery.
It gets confusing. Spoiler alert: avoid H932(T-Mobile), get US998. Verizon, Sprint, & AT&T variants can be Frankensteined, but will require more steps.
For the V30 / V30s / V30+, V30s is definitely recommended (more RAM), but the V30+ is going to be the more commonly available ones. And, the plus doesn't mean anything except for more storage.
For now, it's just broken.
While OLED displays have truly black blacks, they're far more vulnerable to burn-in. As the V30's OLED panel is 18:9, FGO will have letterboxing to fit 16:9 ratio in the form of blue bars - prolonged FGO gaming may inevitably lead to burn-in.
For bootloader unlocking (EXCEPT H932), the WTF exploit works. Also, read the Root sticky.
Pros:
Many official-tagged custom ROMs available.
No need to flash TWRP on every single ROM flashing.
Not OLED, but won't develop burn-in. The 18.7:9 ratio means it'll have the blue bar letterboxing in FGO, however, it sidesteps such issues.
No heat gun needed.
Cons:
Can be (kinda) circumvented by having someone else with a Xiaomi account unlock the bootloader for you, it kinda sucks IMO.
Requires a proprietary unlocking tool. Just like the 1st issue above, can be circumvented by having someone else unlock it.
Only good thing about this : You can wait this 72hr out without using the F1 at all. The bad part? 72hr in real-time, not virtual time.
Decent, but has a wide range of issues, whether it's image retention, backlight bleeding, and/or touch issues. Also, iPhone notch.
No booting to EDL without having authorization from Xiaomi.
Tried it on CarbonROM only, so take with a grain of salt. However, it worsens the visible front camera issue.
You don't know if it's on, & you can't remove it without being unable to use the entire device. Also, the notch makes it difficult to cover.
One permanent method is to open the F1 up & paint over the camera holes. Doable, but not quite recommended as it's a permanent method.
Pros:
Unlocked 3x, locked 2x, all without issues.
Supports stuff like GrapheneOS that actually allows bootloader locking. Haven't tested that one, so take with a tablespoon of salt & please searx it.
Really black blacks. Also, 16:9 means no blue bars in FGO.
Any ROMs flashed will encrypt on 1st boot.
Cons:
A minor hiccup, considering that all other PDAs tested here do have dual SIM / microSD support.
Could have applied this to V30 & F1, but those 2 doesn't have such issues due to their small bottom bezels. On the Pixel XL however, that's just wasted space.
The A1, despite having roughly the same bottom bezel size as the Pixel XL, evades this issue by virtue of capacitive buttons.
In short, flashing recovery after the ROM is a must.
While it'll be more secure, issues begin when booting to TWRP to delete some system stuff - TWRP can't decrypt if a screen lock is set up.
While OLED displays have truly black blacks, they're far more vulnerable to burn-in.
Basically, only 4 Pie ROMs & 3 Q ROMs available & updated, counting /e/(non-XDA).
No volume up + power button to recovery. Go to fastboot & navigate with volume button to recovery.
Avoid the 32GB unless you don't use a lot of apps. And, lack of microSD means plugging an OTG drive for every ROM flashing & backing up media (without using the internet).
You won't know whether it's on / not. Haven't tested on removing it though.
Pros:
While it's not OLED, at least it won't develop burn-in.
Well, that can be counted as an advantage against the F1, I suppose?
Cons:
It's quite slow & do not expect games to work well here.
PUBG tops out at Smooth / High, if anyone's asking.
Since MIUI 11, A1's stock firmware availability is questionable, and can only be accessed via 3rd-party sites. Might change in the future, but who knows.
Recovery has to be flashed on each ROM flashing. However, some recoveries do bypass these issues by having a recovery survival script.
You won't know whether it's on / not. Haven't tested on removing it though.
Can't be remapped & is prone to accidental touches.
Warning: Flashing TWRP zip on stock Pie causes "system is destroyed", and, while it boots to system, recovery reboots is basically impossible. Therefore, start from stock Oreo, and install TWRP from there.
Pros:
While it's not quite as simple as fastboot; it works. Also, no account requirement of any kind for unlocking, just install the unlocker apk, read / swipe through the EULA & warning, connect to internet, & bootloader is unlocked... if it succeeds.
Disclaimer: After bootloader unlocking, warranty for this PDA is void... unless you live in a region that doesn't allow bootloader unlocking to be the reason for void warranty.
The main camera has to flip to the front for selfies. In my personal opinion, this is a plus because, with a flip camera like this (pop-up selfie cams do this too - they're just dedicated hardware tho), I can exactly know if that cam is on.
Guaranteed to last a full day. On VERY light usage (mostly a tiny bit of solitaire), I top out at 5 days, from 80% to 20%.
19.5:9; but avoids FGO's blue bar letterboxing burn-in issues. And, there's no notch.
I don't think I'll use the dual SIM, but it's a nice to have feature, considering that you don't need to sacrifice the second SIM slot to use a microSD.
190g. Doesn't feel all that heavy to me, but it's a nice break from the V30's lightweight feel.
It's there. Not quite close to V30's performance, but it's there. Not the best positioning though...
Fast enough, I say. Though, since I can't really feel the performance gains from 835 > 845, I am NOT really feeling the 835 / 845 > 855 gains.
Any ROMs flashed will encrypt on 1st boot.
The stock ROM can be literally flashed in TWRP just like a custom ROM would. A hassle when upgrading firmware, but at least the stock ROM is decent, aside from Go-ogle & Facebook integration.
Above the volume rocker. Sure, it's not an essential addition, and it's not easy to press due to the positioning, but at least it's programmable. However, this is true only in OmniROM & the stock ROM.
Other ROMs (on Goolag variant) will invariably set this button to activate Go-ogle "assistant", with no options to change it whatsoever.
Cons:
Easy to wreck camera, glass sandwich build. GizmoSlip has a video about this, with the PDA succumbing in the 9th drop, where it landed face down (full details at 7:26).
TWRP recovery has to be flashed again after ROM flashing.
Currently, counting 6 officially supported custom ROMs (Havoc, Bliss, Omni, Lineage, AOSPExtended, Arrow) & 2 custom kernels that's probably not developed any further. Not as miserable as the 2 mentioned earlier, but still. The thread isn't as full of activity as even the V30.
As for the rant regarding ROM / kernel building, that's for another time.
While there's "fastboot oem asus back", it's unofficial & may get blocked on future stock OS updates.
As much as I like the heft, get ready to pull up the pants. This device being the cargo may drop loose pants.
No "turned-off" blacks, so no power savings can be gained with dark theme.
It's volume up + power to fastboot, then navigate with the buttons to recovery. Minor convienence issue.
It's not fun to randomly reboot while doing something (like watching a video) on the 6z. Worse, it can potentially lead to bricking.
Example of my case : When I watched some random video, I would see my video flicker a bit, then the entire screen turns off & my speaker emits a loud static noise. Sure, it turned off after a long press of volume down + power button, but it sucks.
Moral : I think the Zenfone 6 may be the only device I'd recommend buying as a brand new device & test in stock ROM for a while.
For my unit, when I activate the camera, this PDA will invariably glitch out & reboot, regardless of whatever ROM I'm using. That, or it randomly stops while the camera app's open. This could just be my unit, but since this is an issue that could affect others, make sure your camera works if you plan on using it.
This is a massive flaw, especially considering that ASUS' unlock server isn't that reliable lately. While it is mild compared to Xiaomi (it doesn't require any account), it still is a flaw.
These devices lack TWRP support, preventing easy custom ROM installation. Because of this, GApps will always be included by default (OmniROM kinda sidesteps this issue by including microG). The Poco X2 is an exception to this rule as it has an unofficial TWRP.
As for the devices that receive Android 10 update - that's fine & is excluded from this point. However, the Pixel 3 is an exception to this rule, as it follows the full A10 implementation.
Despite the awesome hardware (and the XDA shilling), there's no bootloader unlocking. Skip unless you absolutely have to have the great hardware & don't mind being stuck with stock.
Because it's not Android, obviously. Also, you can't avoid any surveillance program Apple gets to add to their iStuff, as long as you use iStuff.
No bootloader unlocking.
Only if you do mind waiting for at least 72 hours (and possibly longer) to gain bootloader unlock authorization.
Only if you can't stand being unable to relock bootloader.
One Asus PDA I'd recommend avoiding is the Asus 5z. All of its custom ROMs currently are Goolag-only.
In short, looks, feels, & acts like a V30, but is a different beast from the regular V30s due to different RSA encryption.