Should we check that `flux.units` and `uncertainty.units` are the same? `Spectrum1D` expects them to be the same (which is a sensible design decision, I think).
I can easility imagine cases, where the input table has e.g. fluxes in `10^-7 erg/s/cm^2` and uncertainities in `10^-9 erg/s/cm`. This is not a place to convert the units, but we could issue a warning if they don't match.