The problem I have with a FP1U on Android 4.2.2 that there are no security updates.
Whilst Android 4.4.4 is up to date with security.
May I ask why you don’t upgrade to Android 4.4.4 ?
As to 4.2.2 versus 4.4.4 – I get the most battery life out of 4.2.2. However, I am now mildly optimistic that I might have finally figured out what caused the major battery drain that I kept experiencing under 4.4.4. One or two weeks more and I might feel certain enough to explain it.
I have no personal experice on the question yet
I read here (many times) that Android 4.4.4 was unstable over FP1/FP1U.
If it’s like my FP2 with LineageOS 15.1, which reboots maybe once a day and crashes on GPS… I may not advise something else than an official system.
Sure security updates are appealing, but on device with a closed source and not isolated baseband, it looks like a lot of efforts for not so much results anyway…
That being said, to use the phone as a phone, with only Free softwares, it looks reasonably safe (the IceCatMobile is updated for instance).
I can say with certainty that using 4.4.4 was a flawlessly stable experience for me, too. The issue I mentioned above was my sole problem with it (and I now think it affected me for special circumstances).