I almost totally trust the decockers on my guns. Almost. I make sure the gun is pointed in a safe direction while decoking ... just in case. Anything mechanical CAN and WILL eventually fail.
However, I have never personally had a discharge from using the decocker.
IIRC, the original Ruger P89 Mark I had occasional failures until Ruger fixed the problem....but my memory could be wrong....its not as good these days as it once was.
Anyway, I've never seen this failure, period. I trust all of my decocker pistols 100%. The key, as always, is GUN SAFETY. Keep it pointed in a safe direction, and if gosh-forbid a decocker fails at least no one gets hurt.
I own several Walther P99's, all with decockers. I like to practice in DA as well as SA mode at the range, so it's not unusual for me to decock the pistol dozens of times during a session. I've never had a gun to fire in several years of range visits.
3 decocker guns - 2 CZs and a Firestorm .380, and the decockers have always worked fine, but as others have said, always decock while pointing in a safe direction.
Supposedly, some CZ52s can do this. Otherwise, no way. So if you have a CZ52, don't rely on the decocker unless you are at the range, with the gun pointed downrange.
I know you're not supposed to ride the hammer while pulling the trigger to de-cock.
Never with my Taurus PT-92 and any Beretta I used with on. You guys got me nervous now. I'm going to start riding the hammer like a 1911. Ironically I was going to post this question before I saw this thread. Does leaving your gun Decocked and Locked for long periods cause any extra strain or wear to the pistol?