I think there is a decent amount of cool theming and design in the synthesist, which is heavily hampered in 1e because its' built out of a class which was trying to do something very different. If designed from the ground up, I think it could be both balanced and unique.
One of my most hated custom rules in PF1 was critical success/failure on skill checks. So often I have found players playing with such rules and not even knowing that it's a house rule. It creates non-sense problems in PF1, such as my expert horse rider who spends their life horse riding and has full ranks in ride and skill focus still falls off their horse every 2 mins(20 rounds) because of a 1 on a die roll.
Now with critical success/failure being codified in the core rules I'm rather concerned some similar situation could occur. Please please be careful with how you handle skills with regards to failure/success, especially with non-single instance skills like ride. I ride my horse for hours at a time, not seconds at a time. It's not compelling, interesting, or fun when an expert fails regularly. Comparing a game with wizards and magic to reality is often silly, but for skills at least I think it can be valuable. A real world expert horse rider would only fall off a horse when many many unusual situations are arising simultaneously (Circumstance penalties).