One thing about playing medievally is that if you use medieval instruments for authenticity, the audience will think, "So that's what medieval music sounded like!"

But medieval audiences didn't think that. They just heard music. Like we do when we hear modern instruments playing medievally.

So from the standpoint of psychological aesthetics, medieval instruments are less, not more, "authentic."

But that's only for medieval music. For postmodern music, they're fine.