As of now, can a Canadian cross into the US by land or otherwise with just a NEXUS card or does one always have to show one's passport also?
A NEXUS membership card may be used as an alternative to a passport when entering the United States:
by air when used at kiosks at designated airports; and
at all land and marine ports of entry.
However it would seem advisable in some cases to carry your passport with your regardless, perhaps in case of a visit to another country before return to Canada.