Actually, the power pulley does put less strain on the engine, because a) it's smaller and lighter, hence, easier to turn and b) it's smaller so causes the accessories it drives to turn slower; again easier on the engine. As for the manufacturing quality and balancing issues-I don't have one, and won't, but someone out there does. Can anyone with miles on their pulley add anything?
And about the warranty; this is another mod that can give your service department pretty much carte blanche to void a wide range of coverage: Engine (because of questionable balancing), electrical (fried DC power regulator and such), A/C (low compression at idle). Not sure what else is driven by that pulley, but you get the idea.