Rivnuts have the wonderful habit of going together like a bolt and coming apart like a rivet after only the slightest amount of corrosion gets applied.

That tiny interface between the rivet nut and sheet-metal is no match for the surface area of the threaded joint once there is corrosion involved. Sure you can help it out with good material selection and some anti-sieze but dealing with a spun riv nut is a little more twice as onerous as dealing with a rivet the normal way.