Quoted from RazerX:
While harder clear may be good for keeping playfields from dimpling, it's worse IMHO to see the clear separating from the wood because it's harder. Clear should be flexible to adhere to the wood. Dimples in the wood result in dimples of the clear because the clear is adhering properly to the wood. The wood is to blame when this happens, not the clear.
The clear does not really keep the playfield from dimpling. We tried a bunch of 2 part clearcoats on the pneumatic drop hardness tester at work, they were all basically the same. The tests included Dupont Diamondplate, ChromaClear and the much beloved JC661.
None of the drop tests caused the clear to lose adhesion to the substrate.
Keeping the clear from separating from the substrate is a combo of the substrate having enough "tooth", the "heat" of the solvents and of course proper prep around lane switches and the like.
Where the harder coatings really stood out was in the abrasion tests and the amount of polish the surface could be brought up to.