Mercy/Maw works pretty much only if the board is in your favor to start, and that is a gamble. I haven’t evaluated but it doesn’t seem like you get a favorable board on many three-trophy PvP matches. Additionally, on many of the stacked P2W teams, you now have to deal with three other broken troops if you devour Ubastet. Then what? It’s not like you can easily beat Dawnbringer, Infernus, and Ishbaala.
It’s a part of the game now. The devs introduce insanely overpowered troops and weapons that you can directly or indirectly buy. They repeat acknowledged mistakes regarding traits, spells, and team synergies. They might make token changes, but actual rebalances wait until another problem exists. The previous meta cards lose their appeal, and many fade into oblivion. More than half the mythics today are useless compared to the new meta.
This will never change. There will ALWAYS be broken and overpowered troops that you can buy, but their utility is not permanent. They will be nerfed and/or replaced with new overpowered troops…that you can conveniently buy.
It won’t ever change as long as it works.