I've been installing alternative ROMs on my android phones for almost 15 years. Never did I have any stability issues, on the contrary: the community ROMs often did a much better job than the official ones in terms of battery management, etc.
Not trying to contradict what your say, but my experience from a few years ago was:
* If you don't use a super popular model, you would have very few choices for custom ROMs
* Often custom ROMs are still at the mercy of original manufacturer for certain hardware support -- e.g. they need to release "base AOSP image" or something like that
* Custom ROMs often had random things not working, e.g. NFC not functional or cell/WiFi signal weaker than stock ROM
* Some come with certain crappy preinstalled apps. Sure you can remove them, but still annoying
* Battling SafetyNet was a cat-and-mouse game which I gave up
* Browsing xda-developers forum and following the latest reply of a 10-page, 200-post thread like back in the early 2000s was the only way to get updates of a certain ROM. OTA updates were mostly out of the question
* ... and many more added to this
I don't know how much has changed, hopefully a lot. I don't doubt if you have a phone of a popular model, you can find a custom ROM that does not make compromises and is much cleaner and better than the stock ROM. But these days I simply don't have any time for tasking the risk and messing with these things and worry WiFi might not work in some cases.
