Because they aren't out of balance.

I'm sure the valve timing, compression and ignition timing could be softend to produce a smoother running less powerful motor but I wouldn't accuse the YS of suffering from a balance issue.

I'd be more worried if Harley Davidson started producing v-twins for model airplanes than YS producing singles for motorcycles :-) A lumpy 500cc methanol burning, supercharged, fourstroke single made by YS would slot right into a speedway bike frame.

I tried to run an OS max 40FP once and the vibes nearly tore the vice off the bench, THAT engine definitely had balance issues......