[jakeru]

I've successfully replaced a broken underbody-panel fastening stud using epoxy.

Weld repairing here would take a lot of prep work (removing significant amount of paint and undercoating) and clean up afterwards (re-applying any rust proofing, paint, and undercoating.) If there are gas lines or anything rubber nearby, they would also need to be removed (or at least shielded) when welding for safety.

Considering only a plastic nut screws into these studs, the epoxy has a chance of producing a satisfactory repair. (But still, I recommend being gentle on an epoxy-repaired stud! Use an older, nicely loosened-up / broken-in plastic nut, and the minimum amount of torque to prevent the underbody plastic panel from rattling.)