Agree with Eclairius, also we should mention he's more of an assassin designed to be Viable support, not the other way around.
Like, he'll be a great support but he'll lack the gold necessary to function as a good assassin in Mid-Game due to being that role in the first place.
Also i should mention his Ultimate will reset, but it won't stay reset, "When a champion dies in the X, the last ally to assist also gains full kill gold and kill credit, and Pyke can instantly use Death from Below again for a short period of time." meaning he ccan use it multiple times in a row but can't just use it then save it for later when he get's the reset. Also, the range doesn't matter because Darius deals true damage with his which is much more powerful since you can't build against it.
I can agree with you on giving gold to the ADC, this basically translates 1 kill into 2 which is horrifying, so basically if your planning on taking Caitlynn or Lucian bot lane with their great early games, pyke is probably an amazing support for them.