In setting up deep linking to android and iOS apps, I've made some use of custom URI schemes, and they work great, especially as a fallback when the mechanism that allows maps from
http: fails (as often happens with technology).
However, I've noticed one answerer strongly recommend against creating your own url scheme (and here). To me, it seems reasonable to create my own URI scheme, especially when namespaced appropriately.
I looked up the w3c docs on URIs and the RFC that covers creating new URL schemes (RFC2718) doesn't have any recommendations against creation of one's own URI scheme.
The handful of google results show me arguments against creation of new generic URI schemes, but the situation of app-specific provisional schemes still seems like a very useful one.