The couple of times I've done this, our problem has been hitting the enrage timer. Mind you, that's with tank/healer/DPS from a PvE progression guild and one slot filled using group finder. I think you can guess the problem, but that essentially means this is not content for the casual player - methinks the enrage timer is too tight for puggable.
Don't think the issue is about pug but more about under-geared and low skilled players joining the pugs to do the lvl 55 HM. Once everyone is geared enough, the gear should even carry the less skilled players to do the boss.
By skill, I simply mean player's ability to execut decent dps rotation well enough.