Both auto and semi-auto us a centrifugal clutch to pull away from a standstill. On both, this clutch will disengage below its threshold rpm. So the scoot needs a minimum speed to keep the clutch engaged when going downhill. Any difference between the speeds of the two types will be a difference in the lowest gear available. Auto have a slightly higher low gear than semi so will need to go a bit faster downhill to keep the clutch engaged. On my scooter above about 18 kph.