Lack of PATCH method out of the box. No easy way to provide error reporting to the user for some but not all requests. For example, when a user submits a form and it fails, show them a dialog, but when the app makes a background request to refresh some data, and it fails, don't bother the user about it.

It's not a huge problem, I just end up creating my own wrapper around it that provides all the necessary methods, but it could be built in.