I've always been wary of this since although the US doesn't explicitly track exits, they de facto do via airline info and shared info, etc. (Hence you can get a list of both entries and exits from the CBP website for a given passport.)

So if I enter the US on my Canadian passport (as I must), but put my UK passport in on my return flight from e.g. SFO-LHR, it wouldn't show that as an "exit". Also I would like to use Precheck, and my Canadian passport is the primary passport on my NEXUS (I did add my UK passport under "other passports" on the NEXUS site, but I have never tried to see if I can actually get precheck with it.)