One problem I had is that I often just messed up the buttons, especially in the chords, and there's not a whole lot of feedback. I also messed up some things like letting go of the mouse button too soon by accident. Granted, I never spent that long with it, but it's not very mnemonic and requires some amount of motor skills I don't seem to have (e.g. in Vim "dw" is mnemonic for "delete word" and you get more feedback).

Plan 9 in general seems very much like a system "by the designers, for its designers" with not all that much attention to user-friendliness. To some degree that's also the case for Unix, but with Unix other people took the thing they made at Bell Labs and made it somewhat user-friendly (and even then, its received plenty of criticism for it). In Plan9 that step never really happened.