The selector on them is the worst design I have ever seen. Once the gearbox gets hot it goes into 'magic select' mode.
'Magic select' is a system of gear selection where you change down (for example) and get one gear down, or a false neutral before it, or a false neutral after it, or the next gear below it. Until you let out the clutch you'll be completely unaware of whether you're even in 'magic select' mode or not.
Trust me - I used one of these engines in a race bike.