So, let me respond to these concerns in turn.
Doc Strange is considered the single most powerful hero, and is wildly considered a design mistake. He’s just too strong. Everyone else is at least on a spectrum of strength, but he is a tier apart.
That said, Miles is very strong, and Gwen only a bit behind. The thing is, and this leads into what you are worried about, I never feel like their strength is necessary to enjoy the game. Of the original box heroes, for example, I think they are all still solidly playable, and Captain Marvel is still top rate.
The vast majority of the villains are both reasonable to face with nearly any hero, but also both engaging and scalable - you can use plug in encounters to make them stronger, as you like. There are just a few of them that are considered over the top, most notably Ronan and the final boss of Sinister Motives.
On the whole, as someone who has experience with much of the content so far, I’d say balance is generally significantly better than most FFG stuff. Still some misses, still a couple of scenarios that are too much and heroes that go too far in either direction*, but I’m finding the game to be evolving nicely without being reduced to power creep.
My best evidence for this is twofold: I never find myself thinking “Eh, can’t use Iron Man (or the like), he’s out of date.” or “Rhino isn’t good for testing anymore, he’s too weak now.” Nope, the only thing that makes me want to switch it up is a desire for variety, which is how it should be!
*the only hero that is universally agreed to be too weak is Hulk.