None. Focus issues.
(In the case of Metamorph, its basically an acknowledgment that the questionable elements are already doable anyway; once you can put combined powers in an array slot--and that's been clear at least back from the sorcerer archetype in 2e--you could do something virtually indistinguishable in most cases anyway just by making literally everything that wasn't a power an Enhanced Trait and shoving it in one Array slot. It'd be an astonishingly messy looking character, but barring attributes below 10 it'd be indistinguishable from a Metamorph build).
That's actually a misrepresentation of my view; I think there are conceptual reasons to do them and some approaches to certain types of characters that are next to impossible to do in anything but an incredibly muddy way any other way. I just think that you should be damn good and sure you that's the case, because there is a balance problem with the whole approach, and there is cheese built into the mechanic. This is more a case of thinking you shouldn't kid yourself about that, anymore than you should with a few other concepts, and make sure there's no other way to do it. Its not any different than the fact there are other conceptual builds that either are problems looking for a way to happen or that do things that are kind of intrinsically problematic (telepaths with accurate detect minds for example).I'm perfectly cool with the idea that you disapprove of nesting arrays because, in your mind, the only use is point-shaving.
I think you missed the fact that way back in the day I did let one of my players do the "array of alternate forms with arrays" thing.
I think you're using a term here (valid) in a way I wouldn't for reasons I've explained above. Something can be "valid" and still generally "a bad idea." Full PL characters with complete concealment and subtle blasts are "valid" in a rules sense.I disagree with you that the rules state that it's not valid or that it not being used in the archetypes (which are simple builds that have been repeatedly pointed out as not being polished builds) means something. And again, I think we reached this point within the second page of this discussion, that it's a personal choice.