The specific impedance of ferrite decreases with frequency. They show a (dimensional) resonance frequency. This is not the case with nanocrystalline, the impedance increases a bit less than with root of frequency. But as I write it now I doubt: there might be some capacitive effect of the series laminations.
The other issue with nanocrystalline is that the core is strip wound. Therefore eddy losses limit the frequency. Typically strip thicknesses are in the region of 30-35u and this means that at higher frequencies, the eddy current losses are very significant and there is no alternative to ferrite.
Of course another huge disadvantage of nanocrystalline vs ferrite is cost. Even though the higher Bsat of the nanocrystalline cores allows fewer turns, with less winding loss and parasitics, the reduction in manufacturing costs is usually not enough to offset the increased material cost compared to ferrite and therefore ferrite continues to dominate the industry.
There is some ongoing research into lower cost nanocrystalline material with some of the highest cost constituent materials replaced with more economical alternatives and with reduced strip thicknesses, but it does not yet appear to interest the material manufacturers.
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