Do Apple WiFi drivers have a problem with Asus Routers? My two apple systems claim that my ASUS AC-88U is broadcasting country code '-'.
Asus claims  that they will broadcast NA (for North America). All the other routers in the neighborhood broadcast US.
Is this commonplace -- does Apple somehow flub the code for 'NA', or is ASUS blowing smoke at me here and my unit is in fact defective.
No, it is not so that Apple devices somehow cannot read the country code for North America.
Some WiFi devices comes with a settable country code - i.e. often you can access a configuration page from the router and set the country code to match where you are. For some vendors the country code can be changed with firmware updates. Also it is possible that you received your router via parallel import from somewhere else (China or similar) - and that it doesn't have the NA country code because of this.