Yes, unison does require that you have matching versions on both ends of the synchronisation.
Fortunately, installing unison can be done quite easily without root access. On my synology, I simply downloaded the arch linux arm packages and extracted the 2.48.3 binaries from there. Because the packages in Ubuntu 14.04 are outdated, I extracted the binaries from the arm linux x86_64 packages for my laptops. For MacOS, there are downloadable 2.48 binaries that also work out of the box.
That's good to know. Still, I think it would be much saner if Unison was self-compatible at least within the same major-version. It's an excellent program, beyond the tricky details of setting it up on all your systems.
Fortunately, installing unison can be done quite easily without root access. On my synology, I simply downloaded the arch linux arm packages and extracted the 2.48.3 binaries from there. Because the packages in Ubuntu 14.04 are outdated, I extracted the binaries from the arm linux x86_64 packages for my laptops. For MacOS, there are downloadable 2.48 binaries that also work out of the box.