[Autumn]

So long as you're not hurting their FP sustain, yes you can touch up on their interference resistance a bit, I think the largest issue with Dullahans though is just Soul Rage honestly, because of how you can do some really cheesy stuff with the extra momentum.

Anything else can be attributed to achieving the same thing with a Glykin or Wyverntouched and the same build, any sustain they would have is the same as a Dullahan would have, except not immune to Interference Shell and thats really it. (and you can clear interference these days with quite a number of skills.)

I would say have it so that their interference resistance is towards the FP sustain portion of it, so that they have to abide by the same rules as everyone else when they are interfered (except FP wise due to their gimmick), instead of making any knee jerky responses, the holy weakness can come with it I think and be fine, gives an avenue for basic attackers to explore when faced against a Dullahan.