Works for me. Same situation, my other device is a PS5. Created Larian account, connected both GOG and Playstation, connected to Larian network on PS5, restarted PS5 game to make sure it uploads the saves, then I started the game on Mac, enabled Cross Save (restarted again I think, not sure), then the "load game" button was active and I could play. I played for a bit with no crashes. I didn't test the other way around, loading a Mac save on PS5.
So I guess it's supported, but bugged for you.