I feel he needs a rework, but not because he is bad.
Shen has a monkey on his back called Stand United.
While his kit may need deeper work, I argue that a disproportionately large part of Shen's kit power is spent on that damn ult.
TLDR: I think his ult needs changed. It needs to not be global, and needs to be more forgiving. It just takes up too much of his power budget.
Yes its high skillcap, yes it is gamechangingly powerful, but unfortunately every part of Shen is kept weak.
His Q does minimal damage, his passive has poor base and scaling, his E is long cooldown and punishingly high cost when missed, his shield is low power and poor scaling.
Every part of these abilities are kept weak because Shen's ult is STILL strong enough to make up for it.
Unfortunately his ult had a long long cooldown and is tricky to use. When used correctly though it amounts to free kills and towers.
I feel any attempt to change Shen's kit should start with changing his ult. It will be painful, but it needs to be done.
I'd change his ult by reducing its range cooldown, and channel time. Reduce the range to about 1 screen's length, perhaps 1600-2000 range units. (I eyeballed a range based on the maximum initiation range of Jarvan's EQR combo, or Zac's elastic slingshot. I'd then reduce the cooldown to something very very, very low. Something in the realm of maybe 20-40 seconds. Let him channel it in 1 second. All of this adds up to increase Shen's defensive mobility, allowing him to have a solid role in teamfights as a tank who swoops in to protect his allies in the nick of time. He gets to ult more, but shen actually has to spend time doing leg work to set up ganks or saves in skirmishing or splitpushing situations.
It is a complete nerf, and in a vacuum it looks like a kneecapping, but I feel its a better defined ult that allows for more strength to be put into shens base kit while still giving shen that superhero ninja moment, and with far greater frequency.
Also, thanks for starting this thread OP. I'd love to talk about the tank that left me addicted to rage. Every time I play shen I curse the fact that most of his strength is in hard to notice or access places. I usually feel like an ER doctor. WHY CAN"T I SAVE ALL THESE WOUNDED!