Possible oil contamination of the liquid nitrogen.
I have a concern if the liquid nitrogen comes directly in contact with the ice cream. Liquid nitrogen is formed by pumping the nitrogen gas up to a high pressure then releasing the pressure which cools the nitrogen, eventually liquifying it. The pumps frequently use an oil in the pumps and some of that oil can get incorporated into the nitrogen. There would be ways to avoid the oil contamination and I would hope that such ways would be used.