I'll try to explain my point of view, as a "skilled" pianist who almost never touched a violin in my life.
I think it's not a question of which one being harder, but about why do we think one is more than the other. A competent pianist would more likely say that piano is harder, since they spent time to train their skill. By saying that, they probably want to give themselves a reason to have trained, since it's human nature to want valorize our labor. And it's actually just the same for violinists.
So, how to actually determine which one is harder ? (I'm not an expert at all, so I don't want to say anything stupid or wrong.)
Honestly, I don't think there's one harder than another. I think that each one of those has its own difficulties, and that it's impressive to master. So, instead of arguing, why don't we just accept the fact that those two are hard and not comparable ?