As I understand it, modern wireless controllers use bluetooth or a derivative of it, which induces some lag as the input need to be encoded and decoded by the bluetooth chips. Old wireless controllers are infrared without any encoding, you're literally replacing the cable by a beam of light. But other problems arise from this, such as the need to always face the receiver else the signal is lost, you can jam the system by pointing a light at it, etc.
I have Saturn wireless controllers and have had no problem with them. I heard the Megadrive ones were more finicky though. I think it just really depends on how you use them, as long as you don't keep a light source behind you and stop people from passing between you and the TV you should be fine. The beam is wide enough that you can shake the controller without loosing contact, it's just aiming in the general direction, not precisely pointing as if it were a laser. (think TV remotes, which work in the same way)