I'm seeing some odd behavior when using Artist in the hierarchy; it's like MusicBee is defining artist differently depending on what aspect it's looking at. If I put Artist::artist in the hierarchy, it seems to include all definitions of Artist (guest artist, performer, etc). However, when counting the tracks it only seems to pick up tracks where the artist is a primary artist (or at least not the performer (role) credits).
Take Natalie Merchant, for example: clicking her node in the hierarchy gets all of her solo tracks and 10,000 Maniacs where she's included as a performer. That's 37 tracks, but the count only shows 23, which is how many tracks where she's tagged as Artist. And then in the Files With Tags Missing filter, it shows "No tag hierarchy for: Natalie Merchant (vocals)", etc.
Another issue I've stumbled on is there's different behavior related to sub-nodes when using Node::tag vs just Node. If I put New York::location, I get only the tracks tagged with New York, in both the main panel and the count. If I put New York plain, I get all the sub-nodes. I've worked around it for now by making New York::location a sub-node of New York. I can imagine reasons this behavior might be desirable, but it's definitely a problem if there are no tracks with the Node tag, because then it disappears, even if there are tracks in the sub nodes.
A vaguely related question: can you put virtual tags in a hierarchy? [Update: Actually, that won't do what I want. Still a useful question, maybe.]