The following comments are directed to the ADELTs historically fitted to S61s, as they are the only ones I am familiar with. If there are newer, safer ones, then some may not apply.
There have been numerous cases of accidentally deployed ADELTs, not sure how many in flight, but I know of at least 2 on the ground, and if someone had been standing near they would have been seriously injured.
I have had maintenance personnel describe the scheduled maintenance to these things like working on a hand grenade. I know of one instance where the spring shot the ADELT thru the shop window while in for annual check.
As to effectiveness, if they do deploy as designed, where is it going to be when they come looking for you? 5 miles downwind/upwind? I would much sooner have them homing on me than a cork in the sea.
A PLB is the only way to be assured of finding all pax and crew on a dark and stormy night.
I don't get pissed off that often
Sorry for the thread drift......last comment from me on this.