If anything, the KeyStep proves that polyphonic sequencing is manageable.
I can tell you from having read the MIDI spec, made MIDI hardware, and from having hit MIDI bandwidth limitations that allowing for 3-note polyphony on one or both melodic sequencer channels should be a non-issue, depending on certain hardware (rather than firmware) limitations of the device.
However, interface-wise, it can get weird. Right now, the BSP has all these knobs that can change the pitch assigned to a given step. What do you do when chords are stored in a step? You could shift the entire chord, you could ignore the buttons altogetherin that mode, or you could also get crazy and just shift the low note and erase the others. There isn't one particularly "satisfying" user experience. Off the top of my head, I'd probably do something like this:
In chord mode, touch a step's knob, and the notes that are on for that step will be glowing on the pad grid, with the octave up / down pads blinking to indicate the presence of notes up or down from the current octave. In step length and velocity edit modes, the knobs would affect all notes, and in note edit mode, the knobs would transpose the whole chord.
I'm curious about the KeyStep, but I'd be very pleased if I could also get chords out of the BSP. I'm of course only speaking for myself, but it would push me to buying the KeyStep (I love that it's so compact but has MIDI connectors).