Aluminum and you'll be fine.
Steel and you'll do only fifteen or so before needing a break.
Not quite. With one-piece all steel nutserts, yes, you'll be needing a break quickly, BUT you'll also probably snap your mandrel after a few dozen. They take that much pulling force. I've pulled some M4 stainless nutserts, and it was miserable, even that that small size.
The aluminum ones strip out too easily. Avoid these as much as possible.
Look at my link for the steel ones above. They take around the same force as the aluminum ones, but give you steel threads, and they are far less likely to spin in a sheet metal hole. When pulled, that cone half disappears inside the front half, and the splined part actually expands outwards into the sheet metal (the splines on the one-piece part only start to expand well behind the face, as most of the force in those is compression back-to-front). I pulled about 40 of those in the first hour I was installing them with a simple Marson tool, and that included drilling the holes, and I was certainly no worse for the wear.
And I'll admit, I'm not an expert on this by any means, BUT I learned this much from someone who truly is. I had to repair a hundred or so holes in sheet metal that were tapped for M4 that get used on a regular basis. Drilling and tapping to the next larger size was a problem, because then I'd want to do everything, including the ones not yet stripped out, as I don't want to keep around two sizes of nuts and tools. It would make an annoying repetitive task a REALLY annoying one.
So, I went looking for nutserts and happened upon Bay Fastening. I spoke with a REALLY helpful guy there who seriously schooled me on nutserts, and set me up with the parts I needed, which BTW didn't even add up to their minimum order (they usually sell these by the many thousands, not by the hundreds).
