I've heard this comparison so many times I ran some experiments. A number 8 1.5" coated decking screw inserted into two one by pine boards through the grain by a hammer holds about half as well as one inserted using a screwdriver. One hit to drive the screw is better than several, but a two hit approach (one to set the angle of the screw tip, the second to send it home) was most reliable. Drilling a pilot hole before hammering improves things pretty significantly, up towards 3/4 of the holding power of a driver driven screw.

On the other hand, even very slight misalignment between the hammer swing and the screw can result in failure, and the board was always more damaged by a hammer inserted screw.