Is shen a good tank?

If you can land his taunt, you'll do well with him. Otherwise yeah, there are better.

Absolutely! Shen's a fantastic tank! His ability to control team fights and ulti into them is what makes him so fun and powerful to play.

The answer to both of your questions is yes.

As a tank? No, not particularly. He only has 2 things going for him tank-wise- his short-lived and not very strong Feint shield and the fact that he scales decently well with flat HP (albeit only on his passive damage and Q heal). Compared to the likes of Rammus, Sejuani and Nautilus, he just can't be as outright unkillable.

As a champion who just so happens to be a tank? Hell yeah. Stand United is simply one of the best ultimates in the game. It's the reason I play him as both a support and a jungler- because any excuse to fit Shen on a team is a good excuse. His ult is Just That Good (he also makes a pretty damn good support, due to being one of only TWO manaless champs who can be played in the role -the other being Kennen- AND he has sustain to go with it, which is hypothetically endless).

Shen is a good tank in that he's hard to kill and naturally draws aggro to him (taunt, you know), but he's not a good initiator tank like Sion or Malphite. He's better as a peeler, and as such for team fights it's better to play him if you have one of those champs to initiate.