Byleth has the same issue Corrin had of failing to be that distinct from the rest of the Emblemeers. However, Corrin was a disaster that only had the perfect issue of not being the fan favorites the whole community was begging for from the ballot, and also coming from a highly controversial game that hadn't even gotten a western release yet. Most of Corrin's haters have never played their game... but to be honest, most of Corrin's fans haven't played Fates as well.
Byleth and Edelgard have neither of those big issues. Three Houses is mostly beloved, and fan favorites are a dime a dozen recently across Japan, the United States, and even other regions such as Latin America. So, I don't consider them off limits, and they would not be nearly as poorly-recieved as Corrin was. People seem to forget the context behind Corrin.
Also neither of them would play like Marth, but only Edelgard doesn't resemble Marth in terms of structure, personality, or weapon choice.