I read the same horror stories when I did the research. I bought an HG-93 which some people claimed broke easily, one guy with self described chicken legs broke it three times in one trip.

As others have said if it's properly installed you shouldn't have a problem. I'm a weightlifter and I'm strong enough to spin the back wheel on bitumen roads while climbing with a 39/23 gearing and I haven't broken it yet. Also it helps not to do anything stupid like pushing the power while using the small/small or large/large chainring/cassette combinations.

It is also important to oil your chain. The factory grease (wax) might be okay as a lubricant but it offers zero protection against rust if your chain gets wet (guess how I know this :-)).