a lot of the games have buggy code, yes. In some cases it just happened to work on the real hardware. (In others, it did if you were lucky, but if you ran the game on a different model it would fail)

One of the sound programs a lot of Accolade games use for example checks completely the wrong flag and instead of reading the z80 status, actually does an open bus read.

I wouldn't rule out Zero Wing having some buggy code, but OTOH I can't remember any problems with it.