The concept was fully realized in Vietnam, which was a contested airspace (though not quite a peer adversary) but with a lot of EW and SEAD support going on, and pretty much all of the usage since then has been in very permissive environments (and usually at night).

It wouldn’t work in a modern contested airspace, but has been a useful airplane in low-threat air environments because it is very precise (which is surprising given how unsophisticated the idea of shoving guns of various sizes in a cargo plane is) and has excellent loiter time and rough field capabilities.

But even then, the proliferation of even low-level AA weapons has meant that gunship tactics involve flying higher and higher, hence the removal of 20mm and 25mm guns which don’t have the ballistic performance to be effective from those altitudes.