Initially the student piper learns to listen to the drones (and chanter) to avoid the variation in tone that comes from pressure shifts. Later (and relatively soon), I believe, it becomes learned through the feeling of the arm itself.
Once your lips become strong enough to hold onto the blowpipe for extended periods of time, it's the bag arm that becomes the "weak point." I remember a time when I would get 1-hour gigs, constantly playing, that my left arm would be sore for a couple of days after.
The single-reed instruments (clarinet etc.) apparently use the mouth on the reed to achieve effects. I'm less sure of the other "uncapped" double-reeds such as oboe. But even the "capped" reeds (krummhorn et al) have rests, which no bagpipe has.
But I've always had an affinity for wind instruments, valved or fingered. Strings are a totally different realm. Even where they drone, like the mountain dulcimer.
Dude! We gotta get you a didgeridoo!