We have a level 9 synthetist in our group whom in my opinion absolutely breaks the game. The character is a be-all, 5-attacks, 15' reach, dimensional, invisible, flying mount with over 230 dispensable hit points that can easily single-handed kill any and all opponents (possibly even a 20th level fighter) including his entire party if need be.
As a GM, I can pit anything that I wish against him, including a CR20 Demon. But this is not going to be my approach. Rather, I wish for my group to grow together and for us to recognize the principle of greater change from greater resistance. This is a difficult path for it means that all of us must recognize that there will always be broken things in life and even through we have access to these broken things it doesn't mean that we should automatically reach for them at all times.
The best thing to do in my opinion is to expose the class in context to the game. Better to have all powergamers than just one in the group that way everyone can be like a super sayen (dragonball) and create an even playing field that will be evenly distributed amongs everyone including the GM. So for example, up the CR by a couple. But for this formula to work it is important that all players have the same power, so it may be necessary for the GM to give added levels or magic items to each other player that is not the synthesist.
Speaking of magic items, to avoid redundancy of stacking modifiers, it may be necessary to play a low fantasy setting. Why bother dropping magic items that would give more + and abilities where a party has an inherent abundance of these modifiers to begin with? This is not only redundant but ho hum. Unless you want to just skip the item drops and go straight to the artifacts and I am certain their are better game designs out there that play with godlike characters (aka Warhammer 40k heh heh).
So yeah, communication is the best because once everyone in the group realises what is going on when there is an element in the party that abuses the mechanics of play (as intended by the common factors of Pathfinder). Once the GM and other players can really understand the common denominator of the game, that it is NOT the GM vs players but rather players with strings to their characters while the GM turns his hand crank to display his colourful road-- then can players alike begin to understand the necessary demarcations that make a game more enjoyable through the virtue of its esteemed *balance. Perhaps why Chess is revered for the past 1000 years, not because a knight or bishop can win the game by themselves, but rather that they each have a role and function that forms a balanced environment.
Yes, the syntesist is broken. But rather than to disallow this class, recognize the broken for what it is, a shallow, deprived piece that needs help to rise to our humble level. When you learn to play with the broken, then will you see all (aka majority of 3rd party content) broken content and be better suited to deal with it. ANd take it from a 30-year gamer, it is never easy and you will think that you have mastered it only to find that you have not mastered yourself.
Cheers,
