Email required for registration. - annoying and not privacy-friendly
The Android application is just a badly wrapped web-view which does not perform well and has no form of offline caching whatsoever.
The iOS application is not native, being just a browser container. This means that the UX is quite poor, slow, buttons unresponsive. At this moment they do not provide a decent experience.
No web browser support.
Centralized.
Privacy settings for the server are absent, for instance, you can't control who joins the server.
Features not available out of the box.