This seems like a good description from what I've seen so far with Wagtail. I'll give it a bit more testing. For example Mezzanine CMS has the following advantages: more features out of the box, you can edit custom models through common UI, no need to do anything special to them. There is also Pinax, a completely non intrusive set of starter apps for different scenarios.
So, @register_snippet doesn't seem like a huge overhead. But to tell the editors they need to go to "Snippets" to manage user data alongside adverts, but to go to Settings/Users for other user management, is a bit of a bad navigation design.