The granite tops were a trend that Steel City started a number of years ago that had good and bad points. They were nice and flat, easy to use and worked well instead of steel, and I believe we done to try and reduce costs of steel.
The big problem with them was chipping. The mitre slots in most saws got chipped by the mitre gauges. The chipping was mostly at front of the mitre slot, but proved to be problematic. Impossible to fix without replacing the whole top. Chipping and cracking of the granite grew to be a bit of problem for many, some got broken in shipping, others in assembly and yet others who did not read or understand the instructions and use the top as anvil. The granite top could not be used for a number of accessories that required magnetic attachments, like feather boards and jigs. Those are just some of the issues I was made aware of their may have been others.
Generally the saws are fine with the granite top, you just need to make sure you treat them like granite and not steel. Used ones for the right price could be a good bargin but remember that parts could be an issue since Steel City closed it doors quite a number of months ago, and to the best of my knowledge has not been revived.