The Schuko combination plugs you got are heavy duty type, they're naturally better than most other stuff. Maybe they're even outdoor rated.
I've seen metal strain reliefs, fiber ones (not that bad, I never had one break), good plastic ones and bad plastic ones. The one I'm talking about was a plain thin strap of plastic. When I started tightening the 2 screws (2 turns on each screw, then change again) it first started to bend, then it suddenly cracked.
All the other ones I've seen grip cables down to 3x 0.75 mm2 perfectly. The only thing that doesn't really work is SPT2 zip cord. Most people wrap lots of electric tape around the cord (did that myself, what the heck can you fit to such a device, Euro and contour plugs are not available and I definitely don't like to get a whole new cord, there used to be crimp-on Euro plugs but they were banned since most people just stuffed the strands into the connectors and didn't crimp them, I've even seen very rare rewireable Euro plugs, but for some reason they're prohibited by the Austrian code).
Kopp plugs even have the cord grip flexibly attached to the case, so it can't get lost when the screws are out. Once the screw thread ripped out of the case on such a plug.
Can't remember when I last bought a new Schuko plug, I all take them from old appliances etc.
