Well no not exactly, every study I've found shows they're far more effective than a car with just mirrors. Which does make sense! Having to fully shift your attention away and manipulate your head vs. glancing down both keeps all the things you need to be monitoring at least in your peripheral vision, and is much faster than fully turning your head.
Effectiveness of a blind spot visualization system with a video camera for safe lane changes
Effects of blind spot monitoring systems on police-reported lane-change crashes