Basically, the power BG3 built is "Cap yells 'Avengers Assemble' and his team is buffed for one round." Actually, it's built more on the "______ do this, _____ do that, etc." model, but in either case, it buffs the team for one round, and is built so it really only can be used once per combat at best.
Yes, it does so by doing some curious things with the Favored Environment mechanic. But that's more because Favored Environment is a reasonably limited buffing tool. It provides +2 in a particular circumstance. In this case, the particular circumstance is "anywhere, for one round per encounter".
I don't think it breaks anything. Actually, I think it's cool, and stole it for my Cap expy character.
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After his success co-authoring a murder mystery w/James Patterson, Marvel is now talking to Bill Clinton about doing some comics writing: a revival of Steve Gerber's swamp monster hero. They expect Bill Clinton's 'Man-Thing' will be very popular.