Just be careful with anything cheap "you get what you pay for" typically. I'm not sure about good synthetic brands. My first saddle was a cheap/used one and I can't believe I wasn't killed. I was around 11 or ? I was green + horse was green and the saddle didn't help. I was literally launched out of it several times. The back of the saddle was so weak it would pop up at a sudden stop. Never again will I ever plan on getting another synthetic saddle.
I know people that love trail riding in them because they are lightweight and comfortable.
There are no concerns for the horse with a well made synthetic saddle, and they're about 20 lbs lighter than leather (at least the western ones). We've used synthetics for many years without any problems.
Even if it's new it could still be poorly made. I bought my synthetic as used. But it's also a good brand. Therefore it's an excellent quality saddle. Even used. What brand is the oen you have? I say the good brands are Bighorn(i have one), Cordura, and Abetta. I wouldn't use any other brand if it was me.
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.