@Sydaen Primary role is Support. Secondary role is AP carry. You can play AP on him, However, it is not recommended to play him as AP carry due to the fact that you'll be cooldown locked in that you must fight with your ultimate or you won't do enough damage without it. The shadow image itself is extremely squishy because it takes 30% increased damage from all sources.
In addition to my argument, The Ultimate Shade Form doesn't actually do anything. Shade form doesn't give any sort of utility, it even reduces his survivability because Hour of Light doesn't effect him while he is in shade form. Ap carries usually have 3 damaging abilities with moderately balanced base damage and scaling. Doth'our in Shade Form comes out with only 2 damaging abilities, regardless of having high base and high scaling there isn't a third spell to finish off his target. AP carries should also carry one form of AoE. Doth'our's AoE from Hour of Darkness can not be used in conjunction with his Shade Form's abilities because there isn't enough time to combo.
In Yorick's case, you have the choice of 5 champions to use his ultimate on. If you were to pick an AD carry or Yorick all you would need to do is auto attack people to death. However in Doth'our's case, only he himself can be revived from this ability and he isn't an AD carry so he must rely on his abilities. In other words, cooldown locked.
So in my argument, I believe Doth'our's Ultimate is balanced.
@Zanzig It was intentional for the summoner to use 1 spell with smart cast to get off before going into Hour of Darkness. I intend for players to see the open window to use Shadows Consume to debuff the enemy team.