I'm pretty sure the MND stat plays a significant role in overcoming resistance for white magic debuffs.
It's been a couple years since I raised a WHM up to 40, but I remember a RDM posting on a WHM forum saying WHMs should never cast debuffs when there's a RDM in the party who can do it better because of the higher skillcap. When I told him my WHM had never had a debuff resisted for the last 15 levels of XP parties, he said I was either a liar or my chat filters were on too tight, because his RDM kept getting debuffs resisted frequently. A few other posters backed me up, though, and we realized what we all had in common was that we were decked out in +MND gear, whereas his RDM had to compromise between MND, INT, and MP. On the other hand, my INT was not as fortunate, and I saw several resistances when I attempted to use anything like Sleep, Shock, or Burn from my BLM sub.
The RDM in my static party complimented me wonderfully by subbing BLM, loading more on +INT gear, and throwing out the BLM-based debuffs while I took care of the WHM-based ones. Between the two of us, we had everything under the sun stuck to the mob before the melee could say "skillchain."