A friend of mine married twice to non-JWs. The first one left her for a man and the second one beat her nearly to death and now she's a cripple. That being said, it wasn't because they weren't JWs. It was because the first one was obviously gay and the second one was just a jerk.
The way I see mixed marriages is, and I used to explain this to my non-JW mum years ago when I was single is, that if something is such a huge part of your life then the person who's gonna be the most important one in your life should share in the huge part. Otherwise, you and he are always off on different paths ie you're off to meetings and witnessing and he's doing his own thing, rather than you having the same goals. And face it, being a JW permeates every aspect of your life down to the way you think and feel. For instance, you live like armageddon's on the doorstep so you don't plan for the future or worry about paying bills a year down the track, whereas he feels you both should be saving for a house deposit or improving your earning potential, and if there are kids, they will get mixed messages when parents should have compatible goals.
It's all about compatibility and life values.