Been using the Schaller strap locks for years with no problems. Tried the Dunlop locks many years ago, but found them to be a real faff to install (especially the circlips), and a similar faff to use.
Never tried the rubber washer, mainly because all the guitars I use with a strap are fitted with Schallers.
Though saying that, I'm dubious of the new Schaller S Locks, mainly because of the fact that the strap button is now combined with the screw, instead of the old style separate strap button & screw. What if you need a longer screw? What if the existing strap button screw is bigger than the S Lock's? What if the S Lock screw is bigger than the existing screw & needs the hole to be enlarged?