I really don't know if his family had anything to do with his success, so it seems sort of beside the point. It somewhat reminds me of conversations I've had with people about Talia Shire, or about Lorraine Grey, who are often attacked because they clearly seemed to have gotten their roles through nepotism, but I happen to think both are great in the roles they played, so I don't really care. But then, parts in movies don't have the same rags-to-riches mystique that is usually associated with a new Broadway sensation. So there's an emotional/wish fulfillment quality there, whether it's "fair" or not.
Platt just doesn't interest me. I didn't see Dear Evan Hanson on Broadway. I've seen clips. I own the cast recording and the script. I'm unimpressed (I seem to be the only person who is unimpressed: I think the score is blah and the situation more interesting in concept than realization). I tried watching The Politician, but it's another Ryan Murphy work where we're supposed to be enthralled by some gay sociopath, and this gay man ain't interested.
I find him kinda attractive because he's a cub, but that's about it. In interviews he alienates me, though I can't say why. I was a theater kid. Camp could have been a documentary of my childhood (I didn't laugh when the kid referenced doing 'night Mother because I didn't get it was a joke: when you're a theater kid, you do adult shows because those are the only roles that prepare you for the career you dream of), and yet I don't have very warm feelings about all that, and he does strike me very much as one of those kids.
Somehow, he and Lea Michele seem a pair, and that isn't a compliment, in my book.
Tl;dr. Average talent, oddly unappealing off-stage personality.