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 d...
Kuzzle has no hosted version. In order to use it, you have to install it to a remote machine. This can be cost-effective but it also adds overhead because you will have to take care of the hosting and backend infrastructure yourself.
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.
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.