I’m glad to see others concerned about these balance issues. I think for mythic final forms only 14 cost (one more than legendary) is fine. The team cost limit doesn’t seem like it will ever be increased so using mythics restricts us a little more and while the stats boost is good, it’s not worth +2 cost. Mythics have no additional passives or extra effects, just higher stats. They still die to almost all one-shot moves and they’re meant to be better than legendaries because they’re so much harder to get. I think +1 cost strikes the right balance for that all.
As for the lower forms. I wrote an essay in another thread talking about how I don’t like the much lower cost they come at. It’s especially noticeable on mythics because their lower forms have much lower cost than final form monsters of that quality (6 vs 10 SE, 11 vs 13 leg). If they have a viable moveset then suddenly they are a bit crazy.
Ironically the lower cost of the first and second forms of mythics goes totally against the fact they are meant to be more costly than legendaries in their final form!