Short answer: LineageOS is stable enough.
Long answer:
“Preliminary” implies some beta version, or that something different (supposedly better) is coming later on … which was the case as long as the Fairphone 2 port of 15.1 was in the testing phase and not ready for official builds by the LineageOS project.
For the port to become official, however, it had to meet certain criteria, two of which are …
After a port becomes official, it gets updated every week with automatic nightly builds, and this is the real catch here.
Conceptually, automatic nightly builds of course can’t be considered as stable as Fairphone’s OSes with their official beta tester program.
But in the end in practice Fairphone’s OSes have bugs just as LineageOS does, and Fairphone as well as the LineageOS community do their best to fix them.
But the nightlies of LineageOS are just the way the LineageOS project operates, they are not a preliminary thing before something else comes along.