On GMail I I use labels a lot to categorize mails in a nonexclusive and hierarchical way (about 700 labels in total, between primary and underlying levels) while I do not use folders, because they require unique assignment.

Folders have unique relationships to mails (a mail belongs to just one folder) while labels allow multiple relationships to mails (one mail can have multiple labels).

GMail displays labels in a structured manner (like Proton's folders), managing hierarchy and allowing users to expand and collapse levels (just as Proton does with folders).
It would be very nice to be able to use labels on Proton in the same way as GMail, displaying them hierarchically and being able to expand and collapse levels (as Proton does with folders).