The acid seem to be mostly plot based, in alien just a few drops of acid from the facehugger was enough to melt 2 decks, aliens we see acid have some affect but in other scenes not, like the battle in operations at the end, several xenos were killed but were the floor acid damaged? or the air ventilations? nope.
Alien resurrection: killed several aliens and they didnt melt a single deck, the one that johnner killed and its head exploded: no acid hit christie or the other guy, not acid damage to the stairs, same for several other xenos that they kill, barely any acid damage.
In certain times when the writer/director want the acid is a threat but in action scenes its ignored.