The big disadvantage of trackballs over regular mice is you have to clean them a lot more.

Usually trackballs have three or four plastic divets in the socket that holds the ball in order to create fewer points of contact with the socket well and thus reduce friction; These divets have a tendency to scrape any oils or dirt off the trackball onto itself, creating extra friction, which makes the trackball feels slower and less responsive. You have to pop it out every couple days and clean the divets. Comparing that to a mouse, which you basically never need to think about. That said I still prefer the noble trackball to the pedestrian mouse.