I struggled with the Schallers for over a decade: many failures, constant maintenance. I experienced all of the problems noted by others, and replaced at least a half-dozen sets that wore out in various ways. (I've witnessed bandmates have failures with that system as well). I stuck with 'em because the Schallers were better than no lock at all, and I didn't like the original Dunlop locking system because it wouldn't work with an unlocked strap.
Then I discovered the Dunlop Dual Design system. In short: zero problems in well over a decade. All you have to do is ensure that the lock is fully inserted into the button, and that's super easy.
As noted: if Schallers work for you, then great. However to me, they're an example of when I CAN say "never" (i.e. never again).
