For App Engine Google shifted functionality, increased prices and overcharged. They kill or effectively de-emphasize projects at will. This creates a risk of another parse migration. The open source parse is more reliable as you can switch vendors easily.
User registration with Kinvey is pretty good and usually works without any problems. It's also very easy to use, you simply pass the username, password, email, etc. to the required method and register a callback that returns whether the registration was a success or a failure. Same thing is do...
In order to use kinto you have to download it and host it on your own server infrastructure. While it does help you avoid outsourcing your data to a third-party, it also adds the overhead of having to maintain a backend for your app.
For companies or large teams of developers, where there are multiple projects being worked on at once, you can choose permissions for any developer and decide who gains access to each app on the backend.