I feel like he's middle of the road, and on the side of weak. It seems he is best suited to build up to buff heal/shield, and even with everything setup for that, his shield and heal are unimpressive. He's geared more toward tanking (since his entire toolkit seems to revolve around initiation), but he can't tank very well. He seems to be punished way more when Xayah is not around, but the same isn't so true for her. He is extremely reliant on the team, and for clustery team fights, he's at a huge disadvantage (as far as his get in and get out playstyle)--and there isn't crap you can do to defend a turret. Even late-game, you can barely kill a minion in under six hits or a few hits and your Q W.
Though he is fun. And his kit can be antagonizing, especially for those 100hp disengages where you double shield an ally to give them that 2nd wind they need to win a fight or at least get away safely. I'd like to see a little more range on the E when Xayah is not around, and a bit better item-scaling with his Q/E. He may or may not need a buff to damage.