As noted, rails complicate holster searches. A P226 holster for a classic P226 may not fit a rail model.
Balance of a platform one used to like may be changed.
I am one of those who does not like lights mounted to handguns, for the same reason scoped rifle hunters should carry separate binoculars.
I don't like the aesthetics of forward serrations; I also think they entice careless handlers to muzzle sweep their own hands and fingers more than they may realize. I have heard that FCS were originally intended for race guns, that had mounted optics that interfered with gripping rear serrations. I am not sure what FCS are supposed to achieve on a carry gun.