My opinion is that the MB sealant works well not because it's so great, but that the sealing surfaces on MB's are so well machined and fit so well.
I have used Permatex sealers in various areas of my MB, and they performed great. But you couldn't get the valve covers or intake manifold on a Chevy smallblock to seal for very long with ANY sealant!

Similar issue; the MB secret million $$$ sunroof grease. I discovered that Vasoline seems to work every bit as well for my SE.

DG