One recurring problem with playtesting that can contribute to problems creeping into the game is when you get playtesters who for all intents and purposes have no interest in the game they are playtesting and instead give feedback for the game they want.

Seen this way too many times and seen it kill games in mid development when the designer thinks that they did something wrong and need to change the system, then wonder why its both taking forever and feedback from other playtesters is now negative. Because the rest of the testers didnt sign on for this. And so on.

But overall playtesting will more oft than not pick out any real problems in the design that you missed. Or something the game really does need that you just did not think of.