Not sure how relevant this is, but I recently had to remove the inner race from a BMW rear wheel drive axle that just wouldn't move using any bearing puller available to me. It didn't split easily, I had to cut (with a Dremel) almost through the race and then use a hammer and chisel in the groove. After quite a lot of (hard) hammering it finally cracked. I'd say that's a long way from cracking simply from a moderate blow. It all comes down to the alloy and hardening process used, and I doubt (or hope not) that they would make the epicycle bearings in a MGB to "a lower standard"/more brittle than the wheel bearings on a car.