Some surprisingly small crossings can take WHTI-compliant documents. My wife and I used our NEXUS cards (without our passports) at Noonan, ND (opposite Estevan, SK) a couple of weeks ago without issue. When I tried this last year at the same crossing, the agent wanted to see my passport as well, but not this year.

Down the border at Portal, ND (which is a much more major crossing) they've been more comfortable with NEXUS for longer, but they also have taken FAST for years so even though they don't have a NEXUS-specific lane, they are at least used to the idea of trusted traveler programs.