Doesn't Shen's Passive, Ult, and Shield, and his Role of a tank conflict with a lot of things?
I mean his passive for you to do decent dmg atm, you should build HP, but building lets say an HP only item, lets you take more dmg, but not exceptionally tanky, even if you build HP, your shield doesn't benefit from it in anyways, youre just getting more HP basically except it doesn't stack with your passive. Im not even sure if you mitigate dmg with Shen's shield if his passive will work or not, can't remember but lets hope it does. So basically, you need to build HP and armor or MR, items. Which Health and Armor are Omen's (small hp) then Visage (small HP, and small MR) then there's a banshee (wasted mana and HP+MR items wont exist cause considered OP) getting items that give both hp and MR seem to give low of both and therefore only hurting his tank role and not helping it. Sunfire cape is pretty solid, as it helps Shen's dmg a bit also, but MR is lacking, then it goes with Shen's Ult is really unreliable, (besides the teleport though it can break on dmg) It doesn't work with Shen's Armor or MR, or even his HP, and only scales with AP, which again is dumb to build on Shen, at the current time, all of Shen's Ult HP can be drained very very fast and will not be able to save a Squishy lets say even from a Veigar ult, even without Veigar hitting an AP squishy. Now lets go again for Shen's role as a tank, when you use vorpal blade and you hit a target anyone that hits is getting healed a bit, but youre a tank you shouldnt be hitting him you should be tanking and getting aggro of others, therefore the heal does not benefit shen at all only team mates and unnoticeable at late game. Then there's Shadow Dash, such a high costing Energy skill, unless raised first, and when raised first you considerably role as a good tank is hit, and you are rewarded for hitting a champion with it by getting half energy back, but you are basically punished for not using it against a champion and running away by burning half of your energy or more. Even when that's the case, Shen uses Shadow Dash, he grabs agro, but in return, loses quite a bit of energy and not being able to use the rest of his skills properly. Shield is low cooldown yes, but instead it also means you waste a lot of energy, because its so short on cooldown, instead of waiting for energy to fill to use other skills, people would click the one that's already less energy and less cooldown. I rambled on too much but basically, I might be wrong about some of these things while some are situational, it comes down to himself being a complex character to build.