I wouldn't say this as a general statement. I use a visor, ride with my back pretty low and with a long reach, in the drops a lot and had no trouble seeing down the road. Of course, I also buy helmets with visors and check to see that everything works and is comfortable in my usual riding positions.

Current helmet is a POC Trabec, selected in part for its visor. (Mostly for the MIPS technology but I waited until I could get the technology with a helmet with visor in the right size and color.) I mount my mirror to the visor via a little fiberglass bracket I make to custom fit the visor. It is screwed on with two small screws and nuts. (The Trabec is also VERY comfortable, at least on my head.)

Ben