I could be wrong but I think Jaxys initial point was in order for there to be a key the keyway is created which removes material from both the shaft and the pulley (this unbalances the assembly). If a key of material of similar density fills the entire keyway then the components collectively will still be balanced, or within spec. What the tolerances are for operation I do not know.
The pulleys most of us use are not press on and have a loose tolerance against the shaft. With a short key over time the pulley can vibrate back and forth on the shaft. Will it cause enough wear in our application to matter? Probably not. But I use full size keys.
Press on pulleys likely do not need a full size key AS they were pressed on with a tight tolerance and not likely to twist or vibrate like a seesaw around a short key like a non pressed pulley.
My order arrived in the post today. Gotta say, first impressions are very good. Motors look and feel very nice and the packaging was exceptional. Canāt wait to get these fitted to my board. Just gotta wait for the rest of the parts now.