I am a firm believer in the benefits of proper kotekitae, but is anything we do in training comparable to getting hit by a 93 mph baseball?
I've seen literally about a hundred such bruises on my students through the years. Back at UVa we bought our own freezer in Memorial Gym where we would fill stacks of paper cups with water and freeze them for instant ice application after a hard class. The women in particular were grateful for this bruise-reducing measure.
Having been in the batter's box and having done body conditioning for years, my answer is that the training helps. It is one thing to be in great shape for physical activity. It is another story altogether to be in great shape for contact. Apples and oranges; they're both fruit but they're different. Football players spend all spring and summer on the weights. But they aren't ready to play until they get their contact work in. Until kata queens faced boxers, they never understood how important it was to take a hit.
Yes, it might leave a bruise. No, the average Uechika wouldn't experience vasovagal syncope from this. If anything, I find myself a little concerned that things don't hurt as much today as they should. I find myself banging elbows into the corner of cabinets or shins on the edge of a bed frame and taking chunks of skin out. My reaction is more like "That's going to leave a mark!" I'm not numb to it, but the perception is definitely altered.
The closest thing I ever came to vasovagal syncope from pain came in my Goju/aikido class. My instructor taught in his house in a room which was a shag carpet on a concrete floor. Many students had that familiar lump on their acromioclavicular joint which happens when you take a bad shoulder fall on a hard surface. He wanted to demonstrate this "Matrix" move in a Goju bunkai which is where you grab a side kick and spin the person around by the leg. If you've seen The Matrix, you know what I mean.
See I Know Kung Fu - 1:09-1:13
I had only done it once (along with the rest of class) when our teacher's dad came in the next weekend. He so wanted to demonstrate this move to his dad. He was looking for a volunteer and everyone else stepped back. I did the side kick, got ready to throw myself into the spin (to avoid the knee wrench) and accidentally went the wrong way. A mid-air course correction resulted in me falling square on my shoulder. I pretended it didn't hurt, and it really wasn't that bad. At first... Two minutes later I went into the next room and puked.
Syncope? No. But that was a shoulder separation. They do tend to wake up your autonomic nervous system. Sympathetic blocks parasympathetic. Gastrointestinal tract doesn't want to digest any more. Bye bye breakfast!