They're not safe at all. However, robberies should be the least of your worries. What makes them so dangerous are the number of accidents that happen due to drivers falling asleep at the wheel.

  I've traveled all over on them, more times than I can remember; never had a problem. I do make sure to put all valuables in a pocket on the side next to the window though, and not in a pocket next to the aisle.

Im with coffaholic on this

As a Driver trainer I am not the best passenger to start with

And Knowing how people drive in China !  sleeping on one of these buses is not  going to happen

I will stick with the Train

I've taken one to Shenzhen many times (about 9 hrs.). All good so far

I've taken the Shenzhen to Fuzhou route a few times.  I love it.  I sleep like a baby and it's more comfortable than a soft sleeper on the train.

I only use busses as last resort. At least a train will stay on the tracks when the driver is drunk. Especially during festival times, drunk bus drivers would be an issue. 

i think it's safe,but not safer than trains