I've ridden several Norton race bikes with fixed advance, and one street Commando with it (Hunt mag), and it's pretty much what Triton Thrasher said. More likely to kick back, particularly if also high compression, but otherwise not much of an issue. The stock centrifugal advance is all in at about 3000 rpm, and you don't run below that with a race bike, so no real gain there, except maybe starting. I never noticed any problems with the street bikes either, but that might be different if you were riding in different conditions than I was. The bike might tend to run a bit hotter if you had to putter around a lot at low rpm on a hot day, or chug up steep hills with a load, but not something I typically did. It's not like you really loose a lot of horsepower at the low end from having a bit too much advance.
Ken