I have an Apple Watch, but kept that feature off because I didn't want the possibility of someone else being able to fiddle with my MacBook when I'm near enough for the watch unlock to trigger.

Unfortunately, I would place this in the 'Sometimes but don't count on it' half of the "Things Apple Watch has Proven Itself Reliable For" list.

(The only non-timekeeping items on the 'Always works perfectly' part of the list are Apple Pay and the little button that dings your phone when you've misplaced it.)