So the only efficient and effective soft mass-CC we have is Sundial, and there's a good reason for that. Even if this was immaterial, 6 of 'em would allow one of 'em to pretty much always succeed. Since they're not immaterial, we can produce 23 of 'em (even in-element if need be). The only reason it's not obviously OP is because of its high cost. But as we know, high cost isn't a huge issue for a control deck.
If Dial wasn't in the game, we'd be able to add a few less efficient cards like these, but couple the two and I think we have too much unconditional CC in the game. It doesn't help that this has high HP and isn't a permanent (mixed sources of CC).
Also, a plain ol' Alpha Lion would fit just fine thematically. Emerald Mane sounds like a real eye-sore among the normal creatures in Life.