Good question.
What I've understood is that 6.4 through 6.6 are kind of preparatory versions for 6.7, with relatively few perceivable changes. 6.7 should have more impact and features. I just don't want to fall too far behind, since big version jumps can cause nasty breaking changes.
I didn't notice any meaningful performance differences, which is pretty much what I expected, as Hanaa isn't very demanding on the hardware anyway.
One curious little observation: the newer versions seem to be pickier about having multiple or missing audio listeners (there can only be one active at a time). I had some sloppy code on my TODO list that finally had to be fixed, since it was causing occasional audio gaps during scene changes.
In my experience, shaders are probably the most fragile part, and they're also difficult to test. In practice, I mostly just smoke-test things randomly, and since I'm so used to looking at the game, I can't fully trust my eyes anymore. Earlier this year, several particle atmosphere effects had disappeared without me noticing until a couple of months later. That was due to my own mistake, though, and not related to the Unity updates.



































