The Ruination is an empowered Hand of Gul’Dan from Diabolist hero talents, yet in its design it stands in the way of Diabolist hero talents itself and many mechanics of Demonology that involves the usage of Hand of Gul’Dan.
Let’s start with Diabolist hero tree.
The core design of Diabolist makes us cycle the Diabolic Rituals to summon one of several greater demons, we achieve that by using Hand of Gul’Dan where the each soul shard spend on Hand of Gul’Dan shortens the duration of Diabolic Ritual. But when we summon Pitlord that transforms Hand of Gul’Dan into ruination, it replaces our hand of Gul’Dan making it deal chaos damage and summon 3 wild imps for demonology - here the synergy and pros of Ruination ends.
The cast of Ruination doesn’t count towards the Diabolic Ritual, making the next cycle after Pitlord effectively longer. Ruination has a cast time and has a chaos school of magic, that is prone to be interrupted in PvP rendering warlock completely useless for the duration of said interrupt, which is makes Ruination feels unrewarding for the amount of effort it requires to be casted.
On top of that, Ruination doesn’t work with the duration reduction of Doom.
Hand of Gul’dan reduces the duration of Doom by 3 sec.
Ruination also doesn’t count for the Dominion of Argus mechanics.
Ruination by its design, lack of interaction, cast time stands in the way of Demonology mechanics, and I don’t know who felt that replacement of HoG with castable and un-interactable version of HoG feels good.