The good synthetic brands are fine. You want to find synthetics that are built on quality Ralide or similar trees for western. Any saddle with a fiberglass tree or "fiber-flex" tree can be bad for the horse. These trees warp and crack over time and can be warped right out of the box. They don't often fit most horses very well.
Good synthetic brands for western are Abetta, Big Horn, and Fabtron. Southern Trails' synthetic/leather combo saddles are also very nice. For English, the only two synthetic brands I have used are Thorowgood and Wintec. The new Tekna saddles are supposed to be good quality as well.