Usually I get into PUG groups that complete the mission. Fortunately my toons are so strong so they don't become a liability in the group. They do their share and maybe more.
When you join a normal group you rarely fail. The times I see wipes are either low/mid level quests on elite where people aren't geared for the quest.
E. g. my dwarf cleric joined VON3 on elite at level in 2 different groups and wiped both times. Most of the toons were TR's or veterans, but lacked the dps to overcome the regeneration of the Marut. Melees need anarchic or chaotic weapons and we need 2casters to survive.
I've sometimes wiped on hard/elite raids like Hound of Xoriat, Vision of Destruction and Shroud. Those quests are certainly doable on hard, but you need people who know what they're doing and have prepared for the quests. E. g. in VoD all people need remove curse potions (many) and you need 3 healers to share the burden of healing. People need raise and heal scrolls to save mana. All times I failed in VoD the boss was down to about 5%. When the healers ran out of mana and pots the group wiped.
Hound of Xoriat is a bit different. This is a quest where you fight against time. So everything has to go smoothly from getting aggro from Xyzzy, getting stones to charm the pups and getting more stones to keep pups charmed when the original charm wears out. The trick there is that more and more renders and beholders spawn so you fight to keep them at bay until Xyzzy can be killed. You need to get out to grab more stones and if the group outside gets into trouble then the raid fails. I succeeded on a hard raid once when my paladin got aggro from 6 renders and some beholders on the outside running in the outside circle keeping the aggro. I used lay on hands to stay alive and bought time for the others to kill the mind flayer for the stone needed to kill xyzzy. When I ran out of LOH I died and the monsters moved to the center, but it was too late for them. So my pally sacrificed himself for the group.
The worst wiped I've seen was when an inexperienced caster used a globe of invulnerability too close to Xyzzy so the pups moved through the globe. That meant the charm wore off and we wiped shortly afterwards because we had no pups to damage Xyzzy.
So in raid you can actually wipe from a poor decision by just one member. This is something you don't have control of. More often we wipe because of poor preparation from some. E. g. some people join a raid on hard as their first time play through. That's going to make problems. Casters and healers need to have pots if they play on hard or elite and so on. We also see melees coming to the group with no DR breakers. E. g. you have to have silver + good to have full damage against the bosses in Shroud and VoD. Fortunately you can get silver from an artificer.
Still, I think people doing their best to help shouldn't be blamed if we wipe on a raid. Eventually they get better gear and will contribute more. What I don't accept are those who ignore commands from the raid leader and creates a difficult situation. One example is Shroud part 4 where it's normal to save the last devil in each round so blades can disappear before Harry arrives. Some ignore that and we're in the boss fight with many blades and not having all the buffs we need. In part 5 some finish off the last boss before the casters and healers have got all their mana back and so on. THAT is annoying.
Fortunately this doesn't happen very often. So I think pug'ing in DDO is actually a positive experience. Sometimes you get into a group where you feel you're the really squishy one, but then you can just enjoy how smoothly the quest goes. Fortunately those guys rarely complains because you only have greensteel weapons instead of epic ones. Only once I have seen a raid leader dismissing a player because he didn't have a good enough main weapon.
I have one time as a group leader recommended a player to not play though. The time was when we did an elite quest (around 15-17) and the wizard had 86 hitpoints if I recall correctly. The others had between 200-500.