As a fully-geared tank, sometimes I will PUG HM LI just for (masochistic) fun. About 50% of random groups that I get through groupfinder have severe problems with lack of DPS - LR-5's enrage will kick in with 20-30% of the bosses' health remaining, despite the fact that my HP has never dipped below 70% and none of the group members have gone down. After two or three runs like that, I will announce "I don't think this is going to happen with this group" and then leave.
I have experienced that exact problem. Mechanics perfect...DPS is terrible.