Once you master the art of the 5-person PuG, the ultimate risk is a raid PuG. One-shot the instance, or spend the night wiping? You won't know until you try.
I used to run Hyjal PuG's in late Burning Crusade and got to be the person in charge of arranging healers on Anetheron, explaining where to die on Azgalor, and uttering a hollow laugh at suggestions on whether or not Archimonde was in the cards (answer: hell no). I wasn't around for my guild's Naxx run one of these past weeks, and a few guildies were interested in gearing up their alts, so we thought -- PuG a 25-man Naxx? Why not?