In the past, we could only do so above 3 keV using RXTE (and a bit using XMM, but not quiet that well - for this kind of study, one needs many short observations at different times and this is not the XMM observing strategy). Utilizing NICER, we can push down to 0.5 keV and thus finally, finally, probe details of the disk variability in different spectral states of the source!
Generally and kinda unsurprisingly, things are a lot more complex at low energies!