And... just for fun, I stole the 64GB microSDXC (yeah XC, not HC) UHS-1 card from my Galaxy Note 4 (purchased for GearVR use). Of course, it failed. I should feel lucky the K1 didn't catch fire trying to talk to it, eh?
However, the 16GB class-2 microSDHC card I just swiped out of my FPV quadcopter works in the K1 too. So not sure what the limits are. Apparently, any card that CAN work using the hard-coded ASSUMPTIONS in the K1 mmc driver work, and those that don't, don't...
A smarter mmc driver (which reads the specs from the CID table in the SD card) should probably work with all of them. Perhaps even microSDXC cards too, but can you imagine how long it would take to mount a 128GB card if the K1 decided it was time to fsck the card because of some "dirty bit"?