If they’re going out, there’s not much that any CR respondents can say that can change anything…
…but that’s never stopped all of us commenters before, so full speed ahead :).
Genetics wise, as people have said, there is definitely increased likelihood of recessive Jewish genetic diseases (Dor Yeshorim is about ten times as important, but that still doesn’t catch everything). I do know/know of people who have done it (including my great grandparents) and had happy marriages, but I have no idea how it worked out healthwise.
Otherwise, it’s societal norms- but bear in mind that if the shidduch doesn’t work out, family Chanukah parties may get a shtickel awkward.