Tube creep has been an ongoing mystery. Like you say, I've only seen it on my Schrader tires. To be fair, Schraders are more likely to be found on lower end bikes, so maybe they're just more neglected. And my one remaining bike with Schrader is the one that gets the most neglected, since it's the daily commuter bike. Also, Schrader valves tend to be found on bikes with wider tires. But still, 'tis a mystery.

The Schrader tubes I've had are metal all the way down, so cutting the rubber coating doesn't affect anything. I think they're a bit stronger just because the base is bigger and thicker, but that's speculation.

I think what it boils down is that either type of valve requires a modicum of care to use, and the failures are so few and far between that it's hard to pin down either root cause or any kind of quantitative reliability. Most of us ride whatever was on the bike when we bought it. "The good is the enemy of the good enough." In my case, Schrader is gradually fading away because my new wheel builds have all been Presta just due to availability of rims.