At the time I evaluated, Picasa was simply not as good. It was slower, and had a worse UI. Both in terms of aesthetics (in my purely personal opinion) and simple functionality for tagging, organizing and viewing. It also felt rather pushy about sharing.
Windows Live Photo Gallery may be abandoned, I don’t know, it’s certainly been a long time since it was last updated, so I’d be prepared to evaluate competitors again – but if Picasa doesn’t support hierarchical tags that rules it out immediately.
For F-Stop, though, I would regard hierarchical tags as nice-to-have, rather than essential. This is because I am only using it as a viewer/browser. I will not be performing any organization or tagging tasks on my tablet, the PC is far better suited to that.
I did actually want to have “Person/<name>” tags like you suggest, but I couldn’t get exiftool to do that! It would work for a single tag, but multiples ended up comma separated like “Person/Adam, Ben, Claire” or similar. Not much use. So it’s just a soupy mess of tags; but given that there’s no hierarchical view anyway it doesn’t matter much, the tags are good for searching, not so much browsing. Still, it was important to me that these converted tags are only on the Android copy, not actually applied to the master copy, and essential that the process be automatic.