I think Byleth is a ton of fun, and the more I play with him the more he's starting to click with me - but I can't see a future this guy is top tier, though.
He's too slow to play aggressive, but he can't really force approaches from opposing zoners. His bow is just too reactable. Projectile characters like Snake will give him hell. His cqc seems okay at best. He's got a frame 4 jab which is good for a swordy but he also has a below-average out of shield game and bad disadvantage state so he risks getting mowed over by rush down.
in particular is probably a terrible match up for him. The little rat has projectiles, frame data, and a small body to help him duck under Byleth's aerials.
One the other hand, he has good juggling, great ledge trapping and a very strong punish game. Spacing F-Smash at the ledge seems so good, and he can use Side-B, Fair or Uair to cover jumps - it's much easier to space his lance sweetspots when in a ledge trapping scenerio. U-tilt, Uair, and U-Smash all have really good hitboxes for juggling. They're a bit slow but should be a pain for his targets to deal with. He can also go pretty deep for edgeguards, which is nice when other swordies usually aren't able to go that far off stage.
His tilts are pretty nice to. Their hitboxes are great, just a bit slow. I especially love his down-tilt for its crazy range and good reward, but f-tilt is also a good spacing tool that's hard to short hop over.