It does rolls perfectly. As far as knocking the sled sideways, well
I'm not sure. It does have the two handles to hold it with and as long
as it were held firmly the worse that would happen would be to ruin
your work piece. Having said that, surely it would be more likely to
get knocked out of the slots more than miter bars would. Interesting.
Maybe small blocks on front and back of each edge that would extend
down into the slot but not touch it. That way if something happened to
dislodge the bearings, then the amount of movement would be very
limited. As far as general safety, I don't see it's any worse than the
normal risk associated with working with power tools. If you use the
handles, then certainly your hands are out of the way of the blade.
When I was using it to cut box joints, I was holding the wood to the
face with one hand and the other hand was pushing the sled forward
centered on the sled. That put my hand directly in the path of the
blade, but since the sled passes well over the blade I didn't see a
problem. However, a high enough blade would cut into or through the
sled base and you wouldn't want your hand or what then remained of it
where I was placing it. Guess it's a matter of being careful and
applying some common sense just like any potentially dangerous
situation.