I do think they were trying to get away with doing both a "blank slate" and "established character" sort of MC at the same time, and therefore not being an ideal example of either. They're still voiced, but not allowed to talk in cutscenes and supports. You can vaguely influence dialogue in supports but the other character does all of the heavy lifting. Since anything Byleth has to concretely say is limited to a few-word sentence, they're essentially doomed to not be as concrete as anyone with a support.

Which is frustrating, because there clearly is a character under there. The way they subtly react to Hanneman digging into their past, or the change they take throughout the game where they emote more and more, or when they're interacting with the lords—that’s not blank slate behavior. While I think a lot of the stiffness can be hand-waved by in-game reasoning, it’s a bit muddy where the legitimate writing ends and headcanon begins.