TLDR: Global Ultimate = binary effectiveness = outdated design = bad. Shift taunt to ultimate, make it sexy. Replace taunt with anti-projectile projectile.
Scalings and Energy costs are an issue, but I don't think they're the issue with his kit.
I think his global ultimate is too challenging balance around. Either it's strong and the kit is super frustrating to play against or it's too weak and by extension, so is he. This kind of binary gameplay makes a champion's relative power from patch to patch really inconsistent.
How about you move his taunt to his ultimate, and turn it into an ultimate-quality taunt? I'm thinking something similar to Sona's ultimate, but Shen dashes through (rather than casts a wave) and can catch enemies in a very wide berth. What if he gets %Damage reduction based on the number of targets he hits with it?
I imagine you would need to replace that E slot with a utility ability.
I'm spitballing this, but how about a defensive projectile? What if he could throw a skillshot that would intercept enemy projectiles?
Yasuo wind wall comes to mind, but that one is wide bodied, has a short range, and lasts for a while. What if this one were like a long cooldown Mundo cleaver that could knock an SMD Rocket/Ashe arrow out of the air (Ezreal ult passes through)? A flight of kunai with sealing tags on the back (or just the set of sealing tags) would fit here thematically.
Or possibly a shadow clone in the vein of Zed, except this one could eat projectiles? The body swap component would be a dash instead of a blink, and something would happen to players he passes through. Maybe he slows and damages enemies or shields allies that are affected.
Or shit, why not both? If the projectile doesn't catch anything, let it be an anchor he can dash to.
I'm getting ahead of myself.