Going from 10 (no bench) to 20 (+bench) and the attendant rise of drama queens/trying to find 13-15 good players that didn't have horrendous personalities killed my guild. Flex Mythic would have been much appreciated.
Going from server first guild to "can't even enter the difficulty mode this week because we keep having mini-social-collapses triggered by new members which puts us below the required #" sucked pretty hard, and managed to get half of the old raid core to quit the game, which further increased the attendance issues, since the old Raid Core were the folks that never missed a night (thus the no bench for the 10m).
Doesn't help that there seems to be an abundance of competent Melee players, and low-caps on the # of them you can bring to each fight without fucking yourself, which makes recruiting a pain.