Miao Ying initially looks like she has crazy stats and she is offensively powerful compared to other dragon MOUNTS, but she doesn't have a lord riding her adding power or special rules. Then you have to start looking at the unique abilities. Yes hopping 6" out of combat and healing D3 seems nuts, but she has to abandon a combat choosing to do that and cannot charge afterwards, so you're pretty much locking her out of influencing the game for a turn and you have to have some safe place for her to jump. The fact she survives as a human after being killed as a dragon is mid, because then she doesn't hop so whatever killed dragon her likely just kills human her immediately after. If anything it's bad just because it promotes slow rolling attacks into her.

Being a level 4 with double siggies is great but if you're using her as a level 4 you're not using her as a dragon and she's sitting with slow infantry somewhere. If you shift her to dragon form, she's no longer a level 4 and is likely in combat and restricted in what she can cast. She doesn't get to bring any of the utility magic items Cathay has and she doesn't get you a balloon or sentinel as a special choice, stretching out your points more. She's better than a regular monster, but considerably worse in combat than any of the tooled up dragon lords that actually get played.

The movement and spell selection on her is going to be very skill expressive: there are undoubtedly good players who are going to be able to make crazy moves by exploiting her rules. But ultimately I think it is just too fancy and most of the time the benefit for trying to do some haymaker 3-turn play with Miao Ying is not going to be worth the effort vs. just locking in your two regular flying level 4 wizards who are great value by themselves and much more flexible.

People are overreacting without thinking about what the army will actually look like or trying to assemble a list for themselves.