[Kameron8]

Was the extra momentum they get from soul rage an artifact of pre-gr days like the increased momentum from engineer Overclock or was that something added afterwards?

They've had the ability as long as I can remember, so at least for a few years. I wouldn't call it an artifact or a case of old skills transitioning poorly through Great Reckoning -- it's been busted ever since it existed.

Also, maybe just get rid of their interference resistance in general. They already have the opportunity for insane sustain. Adding an additional immunity to the one counter to that while they have every opportunity to be tanky and damaging more than most other classes just seems silly.

The interference resistance was first implemented because Dev didn't like the idea that an interfered Dullahan would be perma-stunned at 0 FP from their passive. As collateral damage, they can receive nonstop healing regardless of the statuses put into play.

Though I'd like to see Dullahans get a penalty to healing received, implementing that plus a holy weakness just makes them metal Vampires in terms of class weaknesses. There are obviously more subtle issues at play with stats, FP requirements, and armor restrictions (See: Roland's post), so I won't throw weight behind any specific suggestion. I'll just reiterate that their sustain can't be countered through traditional means, and both their offense and defense has the potential to be some of the best in the game, all simultaneously.