With pedal extensions you need to check tightness on both the extensions and the pedals on a regular basis. Your old crank probably stripped out because they got a little loose, you didn't notice, and you rode it like that. If I were you, I would stay away from carbon cranks (esp. FSA) because they use a chunk of aluminum glued inside of the crankarms for the pedals to thread into. This might come loose easier with the additional leverage which your extensions provide.