Comment 17 for bug 1875475

The snap package isn't compatible with the version from macOS homebrew either.

In the end, I just went with the binary tarballs available from
https://github.com/bcpierce00/unison/releases

The reason for these incompatibilities is described here:
https://github.com/bcpierce00/unison/#getting-unison

"Beware that Unison uses OCaml's built-in data marshalling, and that this facility is unstable across versions of "ocaml" (the standard implementation of the OCaml language). Additionally, Unison has incompatible changes across minor releases (e.g. 2.48 vs 2.51, but 2.51.2 and 2.51.3 are compatible). Therefore, you must use the same Unison minor version built with the same ocaml version on all systems."

To make compatible packages for 16.04/18.04/20.04 would require either a newer OCaml to build the 16.04/18.04 packages, or an older OCaml to build the 18.04/20.04 packages.

I do think that the incompatible versions should be reflected in the package naming, e.g. "unison" is a meta-package and "unison-2.51.3-4.12.0" is the actual package. Then potentially multiple versions could be installed and /etc/alternatives/unison could point to the chosen one. But unless someone actually builds these different versions, it doesn't really matter.

There are other more modern filesystem synchronization tools available, like Syncthing.