See the 2.52.0 entry at https://github.com/bcpierce00/unison/blob/master/NEWS.md
The major changes are a new wire protocol, a new archive format, and feature negotiation. In particular, once two machines have been upgraded to 2.52.0 or later, it is possible to use unison compiled with different ocaml versions within a machine (mtaintaining the ability to read archives) and across machines.
While this release is expected to work seamlessly with existing archives and with 2.51.[0-5] peers, backups are always in order. Once this release has been run on a root, the upgraded archives will no longer be readable by stable 2.51 releases.
The 2.52 series, expected to be brief in time, is very likely the last group of releases to support old ocaml. All users are strongly encouraged to being making plans to migrate all systems on which they use unison to 2.52.