I don't think I have any arpeggiator that does transpose; I've always had to manually transpose chords, but I've actually been getting lost for a second when swapping modes from sequence to arpeggio when using the Keystep. I've been so locked in single-finger transposing that I'd arpeggiate chord then try to transpose it with one finger and ruin my note run
It would be nice to have that auto accompaniment type feature with the arp, though I'd rather a range function, something every other arpeggiator I own has but the Keystep does not. I know I can just hold a key from the chord, transpose the keybed, then carefully play the chord again, but this is tedious.
Also, my Mircowave XT's arp will play every keyed-on note I play, meaning I can play an interval then while holding one of the notes down, play the other repeatedly to add those instances to the notes played by the arpeggiator, allowing for a simple melody or bassline to be programmed. The Keystep arp will only play a note once in it's cycle so it doesn't matter how many time I hit, say, an A#2 in my 'chord', it'll only be played once each run. Again, it's something I can achieve easily enough by going to sequence mode (and use a sequence slot to record the melody/bassline).