ITIS is less complete because it only covers species that occur in North America and there are several species of Cypripedium that are native to Eurasia. ITIS does seem to be more current than The Plant List or Kew so I tend to favor following ITIS for North American plants and follow The Plant List for the rest.
Glad you brought this up, Evan. I was looking at the Cypripedium database this morning and thinking it needed to be cleaned up a bit. Yellow Lady's Slipper, in particular, has gone through several reclassifications over the years so obsolete names are common.
