The Chicago Piano was actually a fairly effective weapon - when it worked. Skilled crews were able to minimize the stoppage issues and make it reasonably reliable. Prewar tests aboard Phelps and Clark showed that some crews figured out the voodoo necessary to keep the mount firing, while others did not. I would imagine that Lexington's crews were among those who could given their level of training.

What I have read is that the fuse was the really significant problem. Assuming crews could maintain and fire the mount, the shells often exploded prematurely. One account, perhaps apocryphal, was that the fuse was so sensitive that they often exploded when fired during rainy conditions after hitting drops of water. I'm not sure that I believe that, but it was something that was brought up back then. Some ships disliked live fire training for safety reasons, and even forbade it unless absolutely necessary. The fact that the mount soldiered on, even aboard front line combatants, well into 1943 in some cases indicates that it wasn't mission critical to replace it immediately. It was, "good enough." Not great, but good enough.