What about making Yasuo Windwall blocking only non-ultimate projectiles?

Probably because Braum still get's affected with what ever he's hit with, barring damage from the first projectile.

Because Yasuo is just overloaded and his shield negates like 60% of skills in game. Braum can't as easly maneuver around his shield, while dealing massive dmg, being tanky, heal, dodge everything, crit every aa, with emergency shield that recharges while walking. Also Braum taked dmg from blocked projectiles

Because:

1. Braum still get's CC'd and damaged so you feel like at least your spell did something
2. Unbreakable isn't really the same as Windwall. NOTHING projectile based goes passed Yasuo Windwall while if Braum intercepts a targeted AoE like Jinx Super Mega Death Rocket, or Jinx's Fishbones, the damage in an area still applies (unless it's changed and I don't think it has. I've seen Jinx's kill low health targets behind a Braum's Unbreakable). Same goes with Varus Ult. Draven Q (will always bounce back even if intercepted by Braum). Also any projectile that wouldn't hit Braum doesn't get blocked unlike Yasuo Windwall. Lux W for instance.

because boards is filled with many low elo salty babies that are too lazy to figure out how to counter a champ so they join a circlejerk and complain about it instead

Well, Yasuo can always windwall in response and it removes any projectiles between you and where the wall will be. So it's not like Windwall is always down.