On the topic of painting, I find it very difficult too and painting is actually my main focus I practice since years now. (I find is more difficult with skin, and surfaces which have close to no texture. Scales , fur , hair, sand, wood and other texture heavy surfaces are easier for me)
One thing I'd also still love to learn is how to make airbrush shading look flawless and super smooth. Airbrush shading is very hated and I feel like it's because in 9 cases out of 10 its simply not executed well enough and rather smudged and unevenly placed. Doing soft shading perfectly is a challenge. Mad respect to those who can shade like a boss with this technique, make thier art seem 3D with it and more. Currently I can't wrap my head around it but I hope one day I will.