I think the problem with Byleth is how boring (or virtually empty) their personality is. Having recently been through a 51 hour playthrough of the Golden Deer route myself VERY RECENTLY, I would have been happy for someone like any of the following instead:
- Raphael (jolly large doofy guy who just loves eating and building muscle mass. The "super-friendly dumb guy")
- Hilda (uses her charm to be lazy AF and have everyone else do her chores for her)
- Claude (sarcastic, fun, and funny guy who really cares about the people)
Plenty of others I could name, but instead we get the has-almost-0-voice-acted-lines, blank-canvas-boring-empty-personality protag, and I would argue Byleth is probably the worst case of these kinds of characters. Robin wasn't nearly as bad.
PS: Despite 51 hours of play, I had almost no feelings or attachment to Byleth, but absolutely did to the entire Golden Deer hours, plus a few of the others I recruited: Dorothea, Mercedes, Felix, Ashe, and Shamir. Byleth I just legit did not give a crap about despite me controlling them for many, many hours.
PPS: Just a reminder, I'm just disappointed over getting Byleth, not mad about it. Still thrilled to get 9 unique 3H tracks because the music is so goooooood.