Statistically, the Synthesist is the most powerful class at lower levels. As the levels go up however, it becomes one of the less-useful ones. There are times when being able to transform into a flying tentacled monstrosity will be useful, but ultimately its power evens out over the levels.
If you want to run a game that spans 20 levels, then there's no real reason to disallow it. If you're only going for low levels, then it's by far the most powerful class when it comes to sheer physical strength. Remember though; there are many, many, many things that raw physical power can't overcome without dire consequence. Yes, you can beat the bajeezus out of the King for a larger reward, but there will be a ridiculous bounty on you afterwards. Not to mention that I have yet to see a player successfully use their muscles to detect traps or resist spells.
You should be looking at what Synthesist cannot do.
