My friend had his HKParts extension come apart after a single drop onto concrete during a reload (so the magazine was empty).
Personally, I've had good luck with the Taylor Freelance magazine extensions over the last couple of years. I have dropped mine many, many times on concrete, usually empty; the older versions had some issues with bent locking plates (which was generally an easy fix, just flip them around), but I've not had any issues with the new locking plates.
I have like 6 or 9 of the Gen1s and Gen2s each (I think I have like 12 or 15 of the extensions total, would have to count to be sure), haven't touched the Gen3s yet. The Gen1s came with a ****ty beveled aluminium locking plate, which I wrote to Robin about, as it would bend after a single drop on concrete; one of the upgrades that the Gen2s had was a non-beveled stainless steel locking plate, which were much more rugged, and could take a lot of drops on concrete before it would bend enough be noticeable (which is usually well before it would cause it to not drop free), easily 30 or 40 drops. The Gen3s now have a spring steel locking plate, which I've found to be even better than the stainless steel plates, and I've yet to bend one of the black ones (I've put the spring steel locking plates on all of my extensions); I've accidentally dropped a fully loaded mag on concrete with the spring steel locking plate and it did not bend. The Gen3s also have a more significant shoulder on them for retention on the magazine itself, which makes them even more rugged.
Overall, haven't had any noticeable issues with the Wolff spring, the gun still locks back consistently.
Aaron Marco, AKA Dr. No on this forum, used to be on the H&K shooting team, and is a LEO and SWAT team member that has also had lots of success with the Taylor Freelance extensions, both for competition (he's a GM in production, IIRC) and on duty. I know another USBP guy that's been running Gen3s on his personal guns for awhile, too, without issue (he had busted Gen2s before on the shoulder). There are many threads on the Taylor Freelance baseplates on this forum, if you do a search.