I read vague-like statements about Bloodmoon and Tribunal. What I seem to understand: both expansions are partially playable. So that means the rest is unfinished?
Anyway, my real question is. Does it have any value to the developers, if I go play (now or in the future) the expansions? So I can encounter bugs.
Or they want rather to "finish" both expansions first?

From the Project Release
From the Project Release thread:
And from 'Help Requested' thread:
So basically only Misc Quests from both expansions have been completed.
Personally, I'll try to fix bugs in Tribunal Misc Quests if you report any. Probably not going to touch anything else for various reasons, but maybe someone else will.
Frankly, I don't see much use in bug reports for incomplete questlines - there will be just too many of them.