I am not satisfied with our REST support. Ironic given my role, but there you go. Core's REST support is orders of magnitude better than Drupal 7's, but we didn't go far enough. Despite our best efforts Drupal 8 not truly a REST-first system. Some of that is simple hindsight being 20/20; there's a number of things I would have recommended we do differently had I known 2 years ago what I know now, mostly around decoupling storage and serialization. However, we also did not go anywhere near far enough with hypermedia, in part due to time but also due to our at times haphazard and disorganized development process that threw a wrench into the API work needed to support it and delayed it by several months. We have a roadmap defined for improving that greatly from contrib or in 8.1, though. Volunteers welcome!