In the late 90s several of use bought and tried some steel cased 9mm loads out our issue Beretta 92s. No love at all. About every 4-5 round the case would fail to extract and we had to use a cleaning rod to knock it out.
The loads were the dark green laquear coat steel cases. I did some research afterwards and learned about elasticity.
And about under sized, there is "under sized" to spec, which designers take into account, and there is WAY too much under sized, meaning the round rattles around in the chamber and could cause either inaccuracy, failure to fire because of light primer strikes, or both.