We use rivnuts a bit, and use both aluminum and steel rivnuts. Aluminum ones are easy to install with a manual tool, and easier to replace than steel rivnuts. Steel ones appear to be a bit stronger than aluminum ones, obviously, but sizing the grip length properly keeps installation effort down.
Where it makes sense to, we use a clinch/PEM nut. They can be installed with an impact driver and a bolt, a machine vise, a press, or even a hammer if you're really hard-up (not recommended, but it works...). They are ~1/2 the cost of a rivnut and leave the clamping face of the part nice and flat. Their only real downsides are that they can't effectively be installed into some blind-hole applications such as the middle of a long tube, and require a tighter hole diameter tolerance than rivnuts.
My point is that if you're using rivnuts already it's only a small leap to get into PEM nuts which area cheaper, stronger, and offer different low-effort installation options without the need for any specialized tooling. We use both on our robots.
Here's a shot of our 2015 robot where you can see the use of a few steel rivnuts to anchor a transmission (the rivnut head fit nicely inside the hex relief on the transmission housing) and PEM nuts on the chassis brackets. Steel rivnuts worked well for the transmissions: strong enough, nice thread lead-in, plenty of back-side spacing to fit them, not too many to install, and installed into a plasma-cut and post-machined plate (loose-ish tolerances). The PEM nuts worked brilliantly on the brackets: short enough to fit together, installed two at a time in a machine vise, loads of strength.
In our 2016 our ransmissions were thru-bolted because that made more sense, axle bearings were retained with PEM nuts because of their low profile, the bellypan was anchored with aluminum rivnuts which were strong enough and super-easy to install, and the shoulder tube (that provided a majority of the chassis strength) was anchored to the chassis with 6x PEM-nut fasteners per side.
