That's interesting to note - I was coming back from a friend's on New Year's Day and managed to purchase a single from a South Western Railway ticket machine at Cosham and load it onto my GTR Key smartcard.
Possibly you're referring to ITSO compatibility rather than that of National Rail smartcards (excluding ScotRail)? If I were to present my GTR Key smartcard to a SWR ticket machine, it'd work fine, but presenting a Brighton & Hove Buses Key smartcard at a Southern ticket machine wouldn't - ticket machines will accept other National Rail smartcards (those with the
View attachment 113148 symbol), but not ITSO smartcards in general.
On my one trip up to Newcastle, it did appear I could purchase a very limited set of tickets from Nexus ticket machines using any ITSO smartcard from when I tapped my Brighton & Hove Bus Key smartcard onto it. Then again, from your Nexus example, Nexus ticket machines are
designed to work with ENCTS passes as
Metro Gold Card tickets can also be loaded on them.