Man, I love rivnuts. These are great for bolting sheet metal onto a car in places it wasn't originally intended to go. I generally use them when building splitters, covers to flat bottom a car, or rear diffusers in addition to rods running into the frame. They're also great for simple repairs or for making alternate mounting locations.
Yup, they are a car-hacker's best friend, and once you've got the gadget you'll be surprised how often the solution to a random problem around the house turns out to be more rivnuts :)
Only if you DGAF about your stuff being serviceable in the long term.
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.
I've got some hooks rivnutted onto my pool fence to hold skimmer poles, they've been living outside in the queensland weather for a couple of years with no problems.
One of my favorite tools is my rivnut gun.