I've not had any major problems with running PUG raids. There's always something forming up, whether it's Ulduar 25 or VoA. Typically, raid leaders on my server demand stats from prospective group members as well as achievements. No achievement? No invite. It tends to weed out most of the terribles, but there are still a few here and there that get caught up in your PUG raids because their guild has carried them through all of the content already and they have the achieves to show for it.
I'll always give it a few pulls, but if it's clear that the organization is bad or the tanks/healers/dps sucks then I will bail out. Hell, every raid has its share of issues, even guild-led ones. It's worth giving it a shot if you might be able to get some loot or experience out of it.