Had to have mine replaced under CPO after I had it for about 2 months, so that's been about 3 years ago. Since then I will check it out every month or so right before I wash the car. I have found leaves, sticks and other small debris in the vacinity of the tracks that could very easily get into the plastic gears and cause issues. I typically pull the stuff out or vacuum it out. I will also spray the gear tracks and any other parts that look like they may need lubrication with a silicone based lubricant once or twice a year. I use that because it's non staining and has good water resistance. Don't know if that's the right stuff to use, but so far so good.
May want to consider that infreaquent use could also result in failure. Window seals can almost adhear to windows if not used used for long periods of time. The extra pressure caused by a stuck panel could result in additioanl stress on the mechanism. I wipe down the rubber seals with protectant a couple times a year to keep the rubber plyable and from drying out.
However BMW must have some concerns or why else would they include an allen wrench to manually close the roof with the tool kit, and instructions on how to do it manually in the owners manual?