HUGE RISKS, be VERY CAUTIOUS. I would honestly advise against it. Unless you are in a completely committed relationship that you KNOW is not going to fail eventually.
Your risks include a completely trashed credit for a long time if the relationship hits some rocks and the car payments stop. Because it will all fall on you if it does somehow happen. And a trashed credit can trash your life.
Ask any professional accountant and they will tell you to keep your finances and cash to your desperate accounts until you are married. Even then, think before you leap. Even keep bank accounts separate in case of problems.