On Watchexec prior to 1.18, and on Cargo Watch, the underlying technology used to watch for filesystem changes on Mac is FSEvents, a component of the Darwin kernel.
FSEvents is used because Notify, the filesystem monitor library Watchexec uses, chose it as its backend on Mac prior to version 5. However, that was a mistake.
The author of this page is Félix Saparelli, the original author of Notify. They feel confident in stating their own mistakes, and note that Notify versions 5 and up have since changed this. For unrelated reasons, Félix is no longer involved in maintainership of the Notify project.
FSEvents is not designed for the usecases Notify (and Watchexec) is used for.
Instead, it was designed for backup software like Apple TimeMachine: internally, it is powered by a log of filesystem-level events on every mounted volume. When an application requests a watch, it delivers events regarding that subset of the volume. Due to how the logging is done, it may batch events, provide them out of order, miss some entirely, or even deliver events to an application that it didn’t ask for.
The idea behind FSEvents is to receive a notification that something changed on a volume, and so the application should go and rescan either all or part of the volume it is looking after.
The correct interface to use for Watchexec and most Notify usecases is Kqueue, which is a component of BSD kernels, of which family Darwin is part. Notify versions 5 and up use Kqueue on Mac and BSDs, and so Watchexec 1.18+ (and soon, Cargo Watch 9+) use Notify 5.