As regards decoupling alerts, the Sea King 3A has annunciators just under the cockpit coaming (at eye-line) that flash either P, R or C when the cyclic or collective channels are decoupled (either intentionally or through failure). The same PRC legends are on steady when the mode is engaged.
Seems to work very well without the need for additional audio warnings.
So do the Super Puma L2 and 225 (and 92 ) - at the top of the primary flight display anyway. However as I said, you have to be looking at them to see them.
Whilst you are hunting for the next group of brown people to gun down, your state of arousal is probably fairly high and you are paying attention.
Whilst we are droning along in an endless straight line over the ocean eating and reading the paper, our state of arousal is pretty low and how can we be expected to notice an ickle lightly somewhere on a screen?