I have three contracts, two UYs.
Problems? Not really. More inconvenient? Yes. For example, I made a reservation for this summer, and used 270 points. I had to use 150 points from my Dec UY, because the points expire in November. If we only had a Dec UY, I would have booked the vacation and not worried a thing about it (MS always takes points that are going to expire first). However, because I was booking the reservation with two separate contracts, I had to sit and figure out what nights to book with the Dec UY and what nights to book with the Aug UY. Fortunately, Sun - Th was exactly 150 points, so it made it easy. Otherwise, I would have either lost points, or used more than I wanted. Because the points were banked, I couldn't transfer them.
With that said, if your current UY works for you, then I'd stick with the same UY. It will do nothing but make things easier for you. If it doesn't, then the inconveniences wouldn't stop me from buying separate UYs. If points aren't banked, you can make 1 transfer in or out of a contract once per year. This would allow you to "pool" the points, which is the benefit of choosing the same UY.