Unison 2.40 will crash with:
Uncaught exception Failure("input_value: bad bigarray kind")
This is because of changes in object serialization made in OCaml 4.02.
Unison 2.40 requires an older than 4.02 version of OCaml.

At the moment of writing this, the masked version of Unison 2.45 is segfaulting. This is confirmed by other on the IRC channel. A direct bump is thus not possible without any work.