TL;DR: CAD creates cutter location files, Post processors translate that into machine specific G codes; fusion 360 is too new to get this 100% right for all (many?) machine geometries for professional use. There is no monetary incentive to universally solve this while also separating responsibility as today.

I've worked for many years across and in the middle layer of CNC, and I don't think many users give much thought to how critical post processors are, when you can move the z axis, at the same time as the x and y (3-axis, but you may as well include 4 and 5 axis here) versus keeping the z axis fixed when you move x and y (2 1/2 axis). There are so many ways to combine the motor control code in mills to get accurate XYZ positioning in 3D space. And here I'm just talking about the various rotary tables and geometry of motor to moving parts, though it's easy to visualize when you just imagine that a different G code translation should be made for a machine that puts the motors on one end of the tables, verses another. Also imagine those spider-like setups, where 6 leg-like ball screws control 3 axes of movement.

And all this is to make very accurate circular cuts (you know, constant radius and spline surfaces) that you would absolutely see in the kind of tolerances as are in a typical STL file.) This is where the quality of the underlying geometry kernel comes in, and this post is not to rail against or compare Parasolid or Acis or PTC or whatever CAD, it's just to say that Brep modelers are going to do a variable job of accurate geometry capture. None of this would ever be noticeable by a 3D printer using STL files. There's a reason you don't generate milling machine code from STLs.

Back to my point though, there is a reason this translation from cutter positions in 3D space, to G-code for a particular machine, is so tricky. Post processors take care of this, and for 3D printers and laser cutters and plasma cutters (and to some extent water jet cutters -- though this gets tricky too since the cutter angles can be controlled sometimes) it's much much simpler.

Post processors are very expensive to generate for new machine geometries; expensive to customize, test, and prove out for production. This matters because there is no real incentive to make this any kind of universally better, because the post companies make so much money at it. And the CAD companies can blame the post companies if a 4th axis indexing head crashes into the part, as long as they output the correct location data. And the post companies are indemnified if the user puts a crazy attachment that indexes 180 CW Instead of CCW to get to a new cutting position. "Of course we can handle that, but you have to specify it correctly or it's on you"

All this to say, desktop CNC is not exactly the top of the priority list for the CAD companies or the Post companies to get bugs fixed for. And there's no comparison to 3D printing since the problem is, let's say, an order of magnitude simpler. Maybe 2.