Reverse thrust at the gate was one of the causes of the Air Florida crash at DCA in 1982 -- in that case, they had a tug that was slipping, and tried to use the engines to help back away, eventually getting a tug with chains when that failed. Unfortunately, the snow and ice sucked into the engines resulted in false readings from the engine power probes.

I'm sure it's perfectly safe with tail mounted engines and perhaps some wing mounted ones in the right conditions, but I suspect there are significant restrictions on when and how it's used now.