Hi,
it took some time to find all features of Umbrello because "Diagram -> New" is quite hidden for completely new users and since it opens new tabs with completely different toolbars I assumed that Umbrello would only support simple class diagrams.
The tree view is useful for an overview but maybe you should have diagrams separated from shared data like classes. I would prefer two tree views, one like a file list for diagrams and one for shared data.
Besides a start page or something similiar would be nice which directly offers the types of diagrams which could be created with small descriptions maybe.

Reproducible: Always