Apart from that: it depends if the snapped belt just flies away or if it obstructs/blocks the rear sprocket (very dangerous instant blockage of the read wheel). A chain is probably more prone to that than a belt.
On a m/c it is, particularly when leaning in a turn. An experienced rider should recover, but it won't be fun. Also, other traffic participants don't expect a m/c to suddenly stop in the middle of the road for no apparent reason.
Never had such a drastic mechanical failure, just ran out of gas (on the GG bridge of all places) and had a blown main fuse (fortunately on the down-hill section and was able to coast through multiple lanes of traffic to a stop at the side of the road).