project mainline: security updates and some system components updated independently of OEM after EOL
safety net / play integrity
look & feel is on a higher level than an average custom ROM
years after EOL, majority of apps will still support the ROM, kitkat is loosing support from play services this year, majority of apps target lolipop.
Custom ROM:
fun with tinkering
risks, worries, time spent
more updates
hit & miss -> either better or worse battery life, these are often targeting middle-school boys fanatic about anime, overclocking destroys heat management and battery life, safety net can stop working any day
sketchy hacks to get some services for free etc.
nice hacks to get 80% battery charging limit, underclock, automate tasks
huge fragmentation and duplication of effort that leads nowhere among tons of ROMS, competition among them does not do any good currently as they have no incentive to compete
unlocked bootloader -> less security when it's confiscated; but some people are able to lock the bootloader on a custom ROM.
on one hand increased privacy, but not many people audit custom ROMS, so they pose higher security and privacy risks.
I just scratched the surface, but I think custom ROMS have failed people like me who value having no worries about safety net limiting their contactless payments, cards for public transportation within apps, those who value battery life and love underclocks, professional look instead of kitsch, peace of mind, features out of the box like dolby atmos, additional features from the OEM like gesture screenshots, scrolling screenshots and a lot other features.
If you're going custom ROMs, always go with an official custom ROM. Go with a well - trusted one that is consistent with updates, not an official ROM that may be dropped at a moment's notice.
Always choose devices that have long - term support and are consistent with security patches like the Google Pixels (5 years). Do not choose brands like Xiaomi as their lackluster updates mean that you will receive late security updates, and proprietary drivers for the components in your device will go out of date in 2 years. Your phone will be insecure and cannot be made secure when those 2 years end.