Not all of the Z fighter's moves were used against Cell, and neither were any of Frieza's, so you cannot be oh so sure that Cell wouldn't find one that would catch Buu off gaurd. It's like playing a game of Magic the gathering, where one person has several of the same color of card which are all very powerful and cheap, but his opponent has a lot of diversity. The powerful player is likely to come out on top if he doesn't doddle and just keeps on pounding away at his opponent, but if he gives his opponent enough time to figure out his pattern, it can be circumvented. The regeneration is a big deal, but Kid Buu is an idiot, regardless of whether or not getting anygry makes him more serious. Cell is the same way. So is everyone else. Buu was serious when he was beating on Goku too, and whenever he smashed him into the ground he stopped fighting and bragged by beating his chest. Buu doesn't take his battles seriously unless he thinks he is going to lose, and that would probably be his downfall. Any of the Piccolo Buus however would totally trump Cell in a fight, since they would know what to expect, and they wouldn't just stop fighting and watch their opponent get up and charge again.