I have had a couple of issues with my 2013 XU. My rear brake locked up in the first week do to improper adjustment during assembly (I think) and the whole unit including disk had to be replaced under warranty. A bit later, I had the same issue mentioned in previous post with turn signal relay failing. That was an easy do-it-yourself fix. Other than that, in the 2+ years I've had it, I've ridden about 8K and had to replace the rear tire once, and I think it needs it again. I wish those didn't wear out so quickly, because they aren't cheap.
There are a couple of recalls out on this bike. One for the 2013 XU/FX on certain batteries units that weren't sealed properly. You probably need to have it inspected to determine if its a problem on a particular bike -- mine didn't have one. You should check, but that should be covered independent of warranty, since it is a potential fire hazard in wet conditions. (I know, lithium is weird, right?) The other recall is minor and involves a firmware update to avoid a potential engine stall issue if the position sensor gets too far out of calibration. I don't think that's as much of an issue with the XU as its motor runs cooler than the FX due to the more modest inverter amperage.
Which leads me to my final point about the XU, which is that it is over-engineered relative to the other bikes from Zero. It has the same motor, but the inverter can never push it as hard, so the motor doesn't suffer the heating and potential breakdown issues of their other offerings (the newer internal rotor magnet motors excepted). The downside of this is that it doesn't pack quite as much torque during lift-off, but still plenty for a street bike, and lots more that my old Honda PC800 had.