The vertical bar of the color picker where you select the intensity of the color is too narrow. This is a problem for small tablets used with the bigger computer screens.
With a small tablet and a big computer screen, it is hard to hit in such as small area. It usually takes a few trys.
Students often have a 4 by 5 art tablet with a 20+ inch screen. (Don't ask why! That is just the way it is.)