I wouldn't really say that anything is necessarily harder than others. I do believe that artists in general need to put a lot more hours into what they do to become great, but then again it's pretty much the same with every job. Some people are good with coding, some people have an eye/hands for art. Each job has it's own…
They're all different fields with their own complexities. If there is one that's harder than the others, I'd suggest Technical Artist would probably be the one, given you need a handle of coding, or at least scripting as well as art.
Just to weigh in my opinion the hardest to be good at is going to be a technical artist, purely because by being more technical orientated you have to know waaay more fields beyond the scope of just what you see eg coding/scripting/integrating. Which in turn means keeping upto date on improvements on each of the above.…