[magicalhippo]

Handling non-visible spectrum isn't much of an issue, after all the wavelength used when path tracing can be whatever (some have used path tracing for sound). Though getting realistic data for non-visible parts may prove tricky depending on the material.

IIRC the issue is that if you can ignore fluorescence, then reflection is simply an element-wise multiplication of the incoming light at the wavelengths under consideration[1] with the reflection coefficient of the material at those wavelengths. With fluorescence, that turns into a matrix multiplication, with obvious speed implications.

If only a single wavelength is considered at a time, then the wavelength must change upon reflection, otherwise there's no way for the fluorescence to occur. That can also have performance issues, for example conversion coefficients to/from regular color spaces needs to be recalculated.

At least that's my understanding having worked on a physically-based renderer which did do spectral rendering but not fluorescence.

[1]: using for example binned wavelengths or stratified wavelength clustering.