The problem with magnifiers, when compared to LPVOs, is there is much worse light transmission, since the sight has to go through the RDS first, and then the magnifier. The new Aimpoint duty grade magnifiers have only a light transmission of 75% according to the spec sheet; I have no idea what an Aimpoint's might be, though I've heard that some of the Micros have fairly poor light transmission, which is why EOTechs were selected over them for SOCOM (due to the possibility of having to use the optics under NODs). So, low-light performance of a magnifier and RDS can be significantly worse than an LPVO, which is something @
Steve f has talked about.