Rockery is a gravity wall system, FlexMSE is a MSE wall or reinforced steepened slope system. The excavation, backfill, and construction requirements are considerably different.
It would really have a lot to do with the height of the walls you are considering. I suspect that the "rocks" in a rockery that are thousands of years old may win the durability contest if that is your concern and may do better in a fire as well. Cost has a lot to do if rocks are locally available and the rockery form of construction is common. They would be cost prohibitive in Florida for example. Rockeries are very common in parts of the US and fairly economical if the heights are reasonable.
MSE walls including FlexMSE have their considerations as well and work well in many applications also including seismic conditions. Just depends on what you are trying to accomplish and where the project is located.