I recently did Stronghold runs as the only bird w/ 4 bears. It was not a problem, though a medic for birdie was definitely better.

I've done OL w/ all birds. VERY fast. Only OL group that might be a problem is a non-bird group or one with a bird that can't live. Birds that can't survive OL or don't know to spam pots AFTER you tell them are a definite problem. Other than that, lots of groups work. I wouldn't call any group a problem group unless you've tried it and with good players.

I often find 2-bear groups to be a great pleasure, if the bears are cooperating. And as I've said, multi-bear groups working as a team is quite a sight. In a very good way.