Perfect, thank you for clarifying. That's good news, but it's still a bit of a shame that polling doesn't work - I've tried refreshing cache on demand using rc, but it didn't work - union directories still stayed the same after a file on one of their remotes was modified until set dir-cache-time passed.
It didn't happen the other way around - if a file was modified in a union changes were reflected almost immediately on its remote (google drive).
It may be a bad idea, but I've set dir-cache-time temporarily to 30 seconds - will monitor billing and see if it makes a difference. Otherwise smaller dir-cache-time intervals may become problematic in a team environment.
Please ignore that, it was referring to long dir-cache-time times before this fixed it.