Pixelmator is an awesome app for photo editing, or using vector like shapes and text.
But the drawing and painting tools and user interface are not as well designed as other apps like Sketchbook Pro or Procreate. Those offer more shortcuts to help you stay on the canvas painting more rather than having to tap on menus for every change. And of course, they offer far more brush options, custom textures, and pallettes. Not to mention essential tools like color drop inside lines, layer clear or fill, quick toggles for each layer, etc.
If Pixelmator could keep their awesome photo editing tools and vector shapes and text tools, but implement some of the design elements from those other apps for painting and drawing, this would be the only app I would ever use.