I agree that system-wide tagging could be the righ way to properly integrate all of the installed apps. I have been searching for years of a way to group files, notes, bookmarks, passwords and emails in specific containers related to a project.
I took a look to the project feature itself, but it seems to me very limited (we don’t even have a tab to display all the projects toghether and enter in the one we need). I could see that with some work the project feature can be useful, but I don’t even find the repository to try to help.
I think that both projects and universal tagging could be useful in different ways: as @anon75456558 says some objects (files, notes, tasks, bookmarks…) could be related without me needing them grouped, so a tag can be an economic way of keeping track of such relation. When I want to group only some objects, independently of their context (tag) I could use a project.
This is related: Better integration of different services.