dha
The slack adjuster is to keep the brakes adjusted properly.  The air chambers that accuate the brakes only have a limited amount of stroke.  As the pads wear, the slack adjuster must make up for the wear.  All air brakes have an external slack adjuster.  I assume you are referring to automatic slack adjusters.  

I don't know about the failure rate as a % per year, but they can fail.  A common cause is rusting and corrosion from road icemelt chemicals.  Some of the new salt replacements can be much more corrosive than salt.

You can tell if the auto-slacks are working by checking the amount of travel in the brake chamber pushrod.  The pushrod should move no more than 1 inch when moved by hand.  This check must be done with the brakes released.  This requires the vehicle to be on a level surface, and please chock the wheels!  The human body makes a very poor wheel chock.

Improper slack adjustment is usually listed as the number one reason for out-of-service orders at inspections.

Thanks for the information- very useful.. I was also wondering whether over-braking can force the slack adjuster to open past the specified distance and if so, would this be considered a failure.  Also, is there an accepted maintenance schedule or are these maintenance free?