The problem with clone priest is that there is NO counterplay if you are not playing aggro.
As a control deck i can counter most OTK decks by teching for it, with cards like demonic project etc (cause you roughly know when mechathun is in their hand for example).
With priest, there is NO counter play. They summon a malygos on turn 5/6, even if you polymorph it or anything, they will just have another one later. Even if you managed to get a really lucky demonic project off, they just go infinite lyra shenanigans, etc. etc.
This means that the only thing you can do is hope you highroll AND they don't have psychic scream (lol) and otherwise you just sit there and wait for the clock to tick down, even as aggro if they shadow essence lyra/malygos into spirit lash it's just gg, or they keep resurrecting Zilliax.
Having deck mechanics that can not be countered is just extremely bad game design, something the big names in the printed CCG genre learned decades ago. It doesn't mean that the deck is overpowered, but it does mean that it feels very frustrating to play against. You just sit there and hope you kill them before their vomit combo goes off, unable to do anything else.