Everyone is different. Some sefardi/ashkenazi relationships works great. Some do not. Some sefardi/sefardi and ashkenazi/ashkenazi relationships work out great and some do not. There is not a real rule about this. It would be naive to say that cultural differences don’t matter. Sometimes they do, and sometimes the differences are very challenging. But many people have made a success of these “mixed” marriages.
I would say check into the family; see how the father treats the mother(VERY important), how the siblings interact, what the dynamic is. If it is a warm, loving environment, then the rest can be discussed on dates.