If you look at a diagram of planetary gears, you'll see that the different speeds have very different configurations of gears. Some of them push force through 2-3 gears; I bet those are the ones that have issues at high torque.

The standard answer is that if you want to hammer, the Rohloff and NuVinci are better at handling high torque. It's one of those "get what you pay for" things, unfortunately.