From a couple of examples I've seen, I've noted that some repop rubber items are very durable either.I suspect that the softer urethanes have durability issues.
When VTA redid the drivetrain in my bus, they installed aftermarket urethane engine mounts, and the mounts ended up failing in less than 15K miles. They were a much softer material than you typically find with the aftermarket suspension bushings.
Jeff
I'm assuming that it's because low production rubber is some sort of hand mix, room temperature poured product. On the other hand, I have used pre-made urethane and cut/machined it to size that is very durable. We used it in some quick and dirty cutting dies and it always outlasted the dies and was reused over and over. Of course, that was in compression and not in a shear force.
Not to say either is right!! Or wrong!!
This is more of a "thinking out loud" exercise for fun!