The report on the problems with the US 20mm AN-M2 is on my website,
HERE.
I suspect that few problems seem to have reported with the P-38 installation partly because, as has been observed, it had four .50s in the nose as well ,and because it was more rigidly mounted in its own framework, and perhaps partly because it had a mechanical recharger so if the gun failed to fire, the pilot could pull a lever to recock it and try again (I'm not sure whether the Navy installations had rechargers).
The 23mm Madsen had a significantly lower muzzle velocity than the VYa-23, although a slightly higher one than the NS-23. It had a lower rate of fire than either.
Tony Williams: Military gun and ammunition
website