Custom kernels - kernels that's modified by devs to include more features, I guess? yeah, I guess that's that. For the kernel themselves, it's the program with complete control over the system.
For clean flashing a custom kernel :This assumes you start from a clean flash.
As for dirty flashing a custom kernel :
Else, if you don't have TWRP and/or a way out in case of issues; or if you don't need to use one, I don't recommend flashing unless you are ready to clean flash again.
These are some of the features (or anti-features) that may be gained by flashing a custom kernel.
Still subjective. Battery will still go down quickly when the user's doing intense stuff for hours. By intense stuff, I mean CPU & GPU intensive activities like heavy video editing, playing massive AAA games, & using software written by soydevs who probably can't even write any kind of code without help from some sort of hyper-advanced AI autoprocessor.
Available in some kernels. Examples for the F1 include Seshstation, D8G, & Lawrun.
This feature can be dangerous to the display.
In theory, this can be achieved. Gives you more performance at the cost of diminishing returns (you'll probably not feel the added performance), more heat (overclocking does heat up your device - especially the motherboard), & less battery life (more energy required for the OC)
Generally, the availability of custom kernels wildly vary among devices, so use whichever you want (or don't flash any if you're fine with stock kernel / don't want any more issues). Having any kind of overclocking seems cool on paper, but you should avoid it, especially if you want to have a longer lasting PDA.
For kernel managers, the only real option is Smartpack. Kernel Adiutor is no longer updated, FK Manager & EX Kernel manager requires total submission to Goolag botnet and should be avoided. NGK App could be fine for No Gravity Kernel, but not so for other kernels, and it has a SELinux switcher.
For some devices like the Zenfone 6, you should ensure that the required firmware version matches the firmware currently installed in your PDA.
If you don't trust other kernel devs, you can always make your own custom kernel. This way, you have control over the kernel development.
You don't need a kernel tweaker for anything, whether it's enjoying whatever benefits the kernel you're using, or getting better benchmark results. If you really must use one, at least be on the stock kernel.
If you want your display to last, you should avoid kernels with display overclocking enabled. However, if they have a variant without display OC, it's fine to use (the non OC one).
An example of kernels to avoid (for the F1) is LawRun. In addition to the reddit thread below, I'd like to add a few extra reasons : Source code & GitHub releases hasn't been updated & the only place to obtain the latest build are ad-ridden sites.
Reddit thread for LawRun (old):
Also, here are more lists of what kind of kernel you should avoid :