[Phalanx]

Primarily Adventure/Comedy with an overall horror theme -
Which the whole series can really be put into, perhaps in different order per movie, eg:
1 Horror, Adventure, Comedy
2 Comedy, horror, Adventure
3 Adventure, comedy, horror.
...None of them were exclusively horror, all had comedy when you look at it.
That's why they probably just stuck with "horror", which is the dominant theme factor in all of them.
Throw army of darkness back 20 years from the time of it's release and it'd be considered "up there" with the real scary stuff back in the day, no?
Seems to me that's like what they were going for, more an homage to the "old" stuff that we might find laughable nowadays? Maybe, I dunno...the film was definately a bit of a piss take.
It's overall a story about evil forces that want to demonically possess the human race over time - that's a horror tale, regardless of how they choose to tell it.
Unless you want genres listed that cover up most of the case "horror/comedy/adventure/romance"...etc, just deal with it. If 1 (and especially) 2 deserve the "horror" mantle, 3 is there too.

...In any event, I think your argument regarding what the rating "should" be is a little more than 10 years too late.