Comment by jacquesm

There is a lot more to it than that. For instance, for overhangs and bridges you need fairly fine control of the fan in relationship to the movement of the extruder head. You'll need to do circular interpolation in such a way that the toolhead does not slow down too much or you'll get really crappy corners and seams and other joints can be really hard to do properly if you are just using naive point-to-point moves rather than lots of little tweaks to get partial overlap between the two adjacent paths. And then there is control of the extruder retraction and pressure advance, which are pretty complex and difficult to get right for even a subset of the most common use cases, especially if you want to have a range of speed options. "pretty easy" does not match my experience so far, but then again, I am only using this when the regular slicer can't cope so there is a chance that the problem is me.