I don't think they are too complex - and I regret the simplifying process that went on for demons and devils between 3e and 3.5e.
Why?
Because I don't think every ability has to be used in every encounter. I liked having devils that could be planar bound on one occasion to use as a mobile striker/attacker, but in a different adventure the very same devil could be planar bound by the NPC to provide unlimited Animate Dead, so that the wizard could fill his cavern with the zombie bodies of hundreds of slain villagers.
I think that any design decisions which are made on the basis of "this creature can't use all its cool abilities in the 3-5 rounds of combat it survives" are probably poor decisions.
I agree with delericho about dragons though - spot on there. It would have been much better to give one standard set of dragon statistics, and then provide simple flavour templates for the chromatic dragons (not that I'd be unhappy to see chromatic and metallic dragons disappear for ever, but that's another story). Also, if they were going to give dragons magic spells it might have made sense to fill their slots as far as possible with quickened spells so that they can zap them off in combat without affecting their other choices.