Can somebody explain how unison works over ssh?

I have an always on ARM based NAS device with only rsync installed. I want this to be my master storage device to which everything else is synced. Problem is I'd have to compile unison on my NAS which is a real pain if unison needs to be on both the "host" and "client". So a possible solution was to use rsync only, with

rsync --rsh=ssh -aizuKL

both ways. Do you see any problems with this?