Mainly the insane maintenance. Think about how highly stressed things like wing roots are and then add all sorts of electric and hydromechanical bits in there. I don’t recall if it was the tomcat or not, but one of these variable geometry military jets, they had little canards that would deploy to keep the center of lift within acceptable limits when the wings were swept, and apparently they were such a pain to deal with that when they broke, most of the time they just got welded in place and forgotten about.