So I figured I'd see if I have more luck with necro-posting than the previous poster; I don't think my problem deserves a new topic, it's pretty dumb.
I build my Phantom a few weeks ago, and while it was fine in the beginning, entire rows of keys became erratic after a while, becoming periodically unresponsive or chattering. Since rows and not individual switches were affected, I managed to fix it by sucking off most of the solder on the Teensy joints, and resoldering. One row didn't get fixed though, the uppermost one, so I tried to do the same procedure again at the B5 joint... and ended up sucking up the leg. I guess in that case, the problem was that the B5 leg wasn't properly soldered to the Phantom, so desoldering it on the Teensy side made it come loose.
Can this be fixed without desoldering every switch to access the topside of the PCB? I assume no, but I've been wrong about a lot when it comes to soldering lately.