There is nothing magical about FR4 and other epoxy/glassfibre PCB repair (I am specifically excluding cheap SRBP and exotic substrates).
Consider an ordinary FR4 or similar PCB. Thin copper foil was originally bonded to the epoxy laminate by chemically treating it to get a highly adherent porous oxide layer then applying heat and pressure causing a partially cured epoxy surface to flow into the pores, and thermo-set forming a mechanical and chemical bond between the base lamiate and the foil.
With the right materials and appropriate surface preparation of both the substrate and the foil its possible to epoxy bond a track or pad repair that's *NEARLY* as good as the original foil to substrate bond. The only differences are the difficulty of applying heat and pressure to get a fully cured bond, and the slightly poorer adhesion bonding to a fully cured substrate.
However, in most cases it just isn't economic to do a milspec quality board repair, as the full repair process is too slow, needs too much highly skilled hand labour, and the tools and materials required to do it properly aren't cheap enough to make it worth having them available for one-off or occasional use.
With consumer goods, the average repair shop therefore does the best they can quickly, with easily available tools and materials, and rejecting jobs which require too extensive board repairs relative to the replacement cost of the populated board.