Sklansky has written that Hold 'Em is a more skillful game than Stud and that NLHE is more skillful than limit Hold 'Em; this is because draw-outs occur more often in Stud than in Hold 'Em. Super/System 2 states that PLO is more volatile than NLHE.
However, this is only a hunch of mine, but I believe that at low limits, in fairly loose games, Stud is LESS volatile than HE. This is only a hunch, mind you. I think it has something to do with the fact that Hold 'Em is a community card game; against tight-aggressive players, the community card nature makes the game more skillful...but against a lot of loose callers, I think the game becomes suddenly more volatile.
I'm not sure if a buy Sklansky's reason for why Razz is not spread anymore. It is true that experts can play the game perfectly. However, it is also true that the short volatility of the game is mind boggling so fish should still be able to win in the short run and beat the crap out of the experts's bankroll (in the short run). I think the main reason it isn't spread is that many people find lowball poker to be heretical.