The only slightly worrying thing there is that the latest version of the OS
What's the problem with that? Lollipop is the first version with significant changes (new design, ART, etc.). Even then the changes are tiny compared with the major changes in API in earlier (1.x and 2.x) versions. I would expect adoption to be slower than other point releases in the 4.x series and Lollipop not to be the most common version before the next wave of handset "upgrades".
The most important thing is actually the distribution of security relevant patches. Manufacturers have an understandably limited interest in having to integrate or backport changes into their mods but it is for the regulators to make sure they do their job or fine them otherwise. Users generally don't really care what's running as long as their favourite apps run.
FWIW I stuck Cyanogenmod nightly on my S4 Mini the other day and it's very stable with just a couple of things missing.