Based on a cursory search, propellant-grade nitrogen requires a high purity (99.99%) [0]. I am no aerospace engineer, but I'd be interested to know what the actual requirements are for a system like that to work reliably. If 99.99% nitrogen is actually required, then I'd assume that the corresponding compressor would neither be compact nor particularly maintenance-free.

[0] MIL-PRF-27401

Air often contains water though too which could cause problems with ice at the rquired discharge rates.