These chairs arrived and I can not rave about them enough! They are built well, they are SOFT and comfy, they were assembled very easily, they look beautiful! Would 100% buy again. 
The hydraulics work really well and it is quite sturdy, but, that being said, if you really ripped back far, the chair would tip backwards. I’ve been in single pedestal leg stools before that you couldn’t top backwards in no matter how hard you try, and this isn’t one of them! But it IS quite sturdy and lovely. Buy them!