Does Nasus Q count as an On-Hit effect?

Siphoning Strike is a single-use on-hit effect.

Source: http://leagueoflegends.wikia.com/wiki/Nasus

Yes

He wont use it twice with sated devourer if thats what you're asking, only once.

no

Yes it does. Why is so good on him.

Yes, it is on-hit effect. No, it won't proc with Sated Devourer.

It is not a on-hit at all it is on next attack. The difference it on-hit applies on all autos where as on next hit applies one time on the next auto.