Mentor Roulette is a big gamble.

If I'm not having to defeat Bockman multiple times a day, I sometimes end up in a level 60 dungeon with a tank wearing mostly level 50 job gear, or I'm replacing that tank 40 minutes into the run because the group finally got tired of them.

If I do get an extreme, then it's very likely that anyone in there that isn't a mentor is going in blind. I find that the people that queue for extremes are usually new to the game and aren't expecting something so difficult so they have little reason to watch a guide beforehand.