I think it is anything but clear that Haskell is harder. What may be clear is that functional languages are harder to learn for people who have already learned

I think it is anything but clear that Haskell is harder. What may be clear is that functional languages are harder to learn for people who have already learned imperative programming. But then you're forgetting the massive time investments they already made in that style. Having TAed intro classes at a university, I can say from experience that new students pick up functional style much quicker and are always confused by mutation until they've had a fair amount of experience with it.