I don’t see how this is even a debate.
With the right troops he will cast on turn two and kill two troops.
No other troop is capable of that much damage that quickly. It blatantly breaks whatever form of balance the game had left and should be addressed.
I did laugh to myself the other day when an opponents supposedly divine meta defense had elemaugrim in it. After some lucky cascades for the AI on their first go, reducing my attack a few times, it did more harm than good when ubastet casted 
obviously not a well thought out choice 
He definitely needs to be nerfed, the people who can beat that comp either use it, or they use another OP comp like the goblin team and just loop them to death. The point is you shouldn’t be restricted to OP comps just to even have a CHANCE at not getting slaughtered in the first couple turns. That’s what balance is all about. You should be able to put together your own personal team that isn’t always one of the top 3 metas and still do well based on skill as opposed to having outright inferior characters. One person here suggested buffing the other mythics to balance it but that makes even less sense because then everything below the mythics are even less valuable. What the game needs to do is take it’s focus away from delivering new characters for a little while, and concentrate on taking a look at all of the characters released so far to make sure it’s balanced. Because there are countless troops in the game that have become entirely obselete and nothing gets done about that, they just sit in a dark corner covered in cobwebs, forgotten because they just can’t compare to what’s out now, they can’t benefit any comp and even if they can, there’s a troop that does it way better. For example, remember death? A character who’s supposed to be the physical emodiment of death meant to bring about the apocalypse… Yeah he’s worthless, no one uses him because he’s in no way applicable anymore. I say the devs stop rolling in new content for a little bit to make sure the older stuff is all still viable.
Ubastet, especially on its own, is really, really not the problem.
I have seen a fair amount of Ubastet Teams and they are relatively manageable.
None of Ubastet’s traits is a real problem.
His mythic Trait supports Rakshasa specifically, as opposed to other traits that can easily cripple your whole team.
The problems with defense teams pretty much universally are the AI opponents that can predict crazy chains of matches and trigger lots of traits that go off on a 4+ match.
If you want to nerf something, do something about the AI and how it can predict something resulting in a four gem match that it should not be able to predict (you can see the AI frequently ignoring four gem matches for something that then magically creates one from gems that had not been there or random transformations or such).
There are just so many relatively easy ways to deal with just Ubastet compared to other popular Mythics.