Ok, the least user friendly bicycle rear dropout are the semi-horizontal ones. If there’s a fender installed, and a touring tire, then it’s fiddly to take the wheel out. When putting the wheel back, it’s tricky to make sure it’s centered and has the right position in the dropout. And then the derailer tries to push the wheel to the front, so while you’re centering the wheel it might just pop out, and the gripping washer can easily damage the paint in the dropouts. So many things can go wrong.
Vertical dropouts are much easier: put the wheel in, tighten the axle bolt and you’re done. The wheel is either sitting in place, or it’s not. Easy.