Attempts to emerge unison-2.9.1-r1 on my system fail with an error about "inconsistent assumptions over interface pervasives." The fix was to force an upgrade of dev-ml/lablgtk from 1.2.5 ("stable") to 1.2.7 ("~x86"). Then I was able to build and use the supposedly "stable" unison-2.9.1-r1.
it is stable, the problem you encountered is due to a dependency hell in portage for ocaml packages (because of ocaml compilers binary incompatibility). There's a script to rebuild all ocaml libraries after an upgrade in dev-lang/ocaml/files.