I've actually had the opposite experience, with the few union contractors I've delt with (pipe fitters on one job and electricians on anther). Granted, my experience is limited, but the union guys seem to have a little bit more professionalism about themselves and their work.
I've definitely had issues, but that's common for all trades (lack of clean-up, not wanting to work past 40 hrs, etc.).
Edit to add I work working on the GC side of things.
Having been a subcontractor who worked out in the field before, I can say that my interactions with union contractors were usually negative. Defiant, lazy, not willing to work with you to get the job done over a technicality that your company isn't union and his is, etc.
This post was edited on 11/15 at 12:53 pm