[maxpro]

This is only if you use it hierarchically. No one prevents you from having a tag projects and another tag newproject. Your emails just have both tags assigned and you are good to go

[OJFord]

Sure, but GP's point (and it's something that annoyed me in Fastmail too) is that if you do have an 'hierarchical tag' - which could be in addition to unrelated tags - you don't get the '<sub-tag> is a <super-tag>' that you might expect.

The tags themselves end up in an hierarchy without it affecting the tagged contents.

[dredmorbius]

From a document perspective, yes, you're correct.

From a tagest perspective, and the discovery affordances, there is a significant difference.

Hierarchical tagsets allow you to navigate via tags, affording document discovery by that mechanism.

That becomes more significant as you're working with a larger, richer, and more structured tagset.