One thing about regular rivnuts (I'm just in the habit of calling them that instead of nutserts) is that if unsealed they will leak, both through the hole that the rivnut is inserted in and past the bolt that is screwed into the threads. I did not expect this to be the case when I first started using rivets, but I have a metal box experiment set up where I run a fastener of whatever kind through the bottom and then fill it with water to test for leaks. I was surprised to find that my pop rivets leaked unless I put them in "wet" with seam sealer around the hole, and that the rivnuts leaked the same way (and also leaked past the bolt if I sealed the rivnut itself).
It seems other people have reported their rivets being watertight on their own, so perhaps I'm doing it wrong, but I've always put everything in wet since then. The thing with rivnuts is that you kind of want to be able to easily unscrew anything you run into them, so putting sealant on or around the bolt itself kind of defeats this. For my replacement LED lights on the back of my bus, I wanted to use rivuts and have them be watertight but still allow me to unscrew the lights when necessary, so I used closed rivnuts which are watertight (without glopping up the bolts) unlike the regular kind. Unfortunately, the biggest I could find were for 1/4-20 bolts, so I don't think the closed variety could be used for something like a roof rack attachment unless bigger-diameter ones are available somewhere.
Also, there seems to be at least two different types of rivnut setters available on Amazon, and they take mutually-incompatible bits (I broke off a couple of my bits when first using the tool and unfortunately ordered the wrong kind of bit replacement) but it's hard to tell which is which because some of the tools are knockoffs.