I presuming you are talking about a finger bar mower that was designed to be horse drawn, not an adaptation of a tractor-drawn one, and that your question is as to how effectively they cut the grass.
Well I bought one many years ago and we used it for several years to top our grass and it was very effective. Obviously you have to keep sharpening the blades to get a good cut.
We actually towed it with our Land Rover, but that would have exactly the same effect as using a horse.
So yes, I would say they are a very good bit of machinery.