His kit isn't helping going either AP, either tank.
This is basically the main problem.
Going full tank, you end up doing no damage and getting outscaled by Sion, Shyvana, Garen and CIE while not bringing much more than them even if they are full tank.
On the other hand, if you go with an AP tank build, you end up doing a lot less in a teamfight than a Cho'Gath, a Gragas or a Maokai.
The main problem comes from the design of the character in the actual meta. Shen was meant to be an AP Tank. His ult is supposed to help him apply pressure somewhere while being able to regroup quickly (Splitting). And this is where the main issue comes into place.
If you go AP (like Shen is supposed to be) you lack pushing potential or damage to face a dragon and too many characters can easily outduel you. If you go full tank, you still lack damage and a regular top laner can easily stop you from pushing and join back their team as easily as you do as Shen.
If you go AD, your abilities doesn't help you enough. (Q barely does any damage, the sheild doesn't block much and your ult can be popped easily and the target you are ulting could be killed as quickly.)
Currently Shen is in a bad state. It's not his ability to be played in the current iteration of the game that is a problem, it's more his kit as a whole. I love Shen too, the idea of the champion is really neat. But he needs something to be special.
