NOTAR
I must rely on the experience of others who have actually flown them but I'm told that the weight of the fan mechanism takes some getting used to. It's situated immediately behind the cabin and not easily balanced.
For military uses it might prove advantageous to be able to back into a dark notch in the woods and either land or observe from the hover. However, it will take some practice to perfect. I've also been told that the reaction time is notably reduced.