I know several “mixed” families who seem very happy and content. Its probably dependant upon a lot of variables, though.
What type of Ashkenazic person is the Ashkenazi? (Yekke, Hungarian, Belzer Chosid, Litvak, Yerushalmi etc etc)
What type of Sephardic person is the Sephardi? (Spanish, Yemenite, Persian, Syrian, South American etc etc)
What type of educational culture did each party grow up in?
Both sides would have to be open and accepting (and certainly not condescending) of the cultural differences. Once you have that, as well as an understanding of differences, why not (if personalities, life-goals and Hashkofas line up)?
As with all things in shidduchim, its good to have a Rebbe/Rebbitzen/Rov/Competent mentor who can direct. Hatzlocha