I have installed mopidy-youtube and configured it, but when I perform a search (in Iris) I get these errors:
May 20 13:43:37 pab mopidy[1048]: ERROR [YouTubeBackend-3] mopidy_youtube youtube search error "'NoneType' object is not subscriptable"
May 20 13:43:37 pab mopidy[1048]: WARNING [Core-17] mopidy.core.library YouTubeBackend does not implement library.search() with "exact" support. Please upgrade it.
I also struggled to get Mopidy-Youtube to work reliably (irrespective of the front-end I was using). It has since had a few releases, so perhaps has patched the issue you experienced.
FYI @jjok the idea behind Iris is to support all Mopidy backends (Spotify, as it happens, is the only one with a decent HTTP API). The capabilities that each backend provides governs how immersive it is in the Iris interface, but all backend providers should be at least accessible with Iris.