For explanation's sake:
The Aliens in their omnibuses get taken out by the dozen. It usually takes one bullet to put them down and any gun will do. That's not the end of it, though; there's at least one novel where a human takes on multiple Aliens in hand-to-hand combat and wins. Very occasionally, when the plot demands, the Aliens manage to do some damage.
It's not just the Aliens, either. The whole setting is damaged, because the writers seemed to want to make the setting into every other sci-fi that's ever existed instead of a reasonable, over-capitalistic future version of our own society.