unison doesn't include
unison-fsmonitor for macOS, thus
-repeat watch option doesn't work out of the box. This utility fills the gap. This implementation was originally made for macOS but shall work on other platforms as well like Linux, Windows.
brew install autozimu/homebrew-formulas/unison-fsmonitor
Alternatively if you have cargo installed,
cargo install unison-fsmonitor
Simply run unison with
-repeat watch as argument or
repeat=watch in config file.
Increase file watch limit on macOS