I think there are two major problems with Day:
1) It follows after Dawn
2) People take it as part of a trilogy instead of a standalone movie
Dawn is, overall, completely atonal in contrast to Night and Day. Night is claustrophic, Day is hopeless. Dawn is very optimistic, happens in a mall, funny goofy stuff happening with the zombies, nice ending where the heroes literally ride off into the sunset to find a safe harbor until the crisis ends.
Then you start Day, and you're back in a helicopter, and you see that there is absolutely nothing left of civilization. It's not a promise of freedom, it's a promise of flying as far as the gas can take you, landing then dying. Your literal best hope is that you might maybe just maybe make it to a tropical island and die, alone, the last vestige of humanity.
In Dawn there is hope for the future. All of the tools of civilizations are there for the picking, if somehow you're able to make yourself a safe environment, life will go on as it always did. There are threats, sure, but isn't that what life's like? Night by contrast was claustrophobic. At any moment the threat could burst in and you were constantly worried you might die. With Day death is the inevitability, the question is are you gonna get there because someone will fuck up, because you end up eating your own gun, or you're driven insane by the few assholes left alive and you all kill each others before the zombies even have a chance to get you.
All those movies happen in the same universe, so to speak, but they are not really a trilogy, anymore than John Carpenter's Apocalypse trilogy was a continuation except thematically. People come off the nice happy vibe of Dawn and then they start Day and it's a fucking whiplash of depression, hopelessness and a bunch of cunts arguing together constantly. You don't really have anyone left to root for, because the kind of person you'd want to root for has already been turned into a zombie already.
Night was about how people react in the midst of a tragedy. Dawn was about how they cope and try to ignore the bad things going on around them. Day is about just how low people will go once they know it's all over.
That's where I have to disagree the hardest. The effects and cinematography of Day are far superior to Dawn's. You fucking
feel like you're in those mine shafts, the whole place is drab and depressing and that's exactly what it's supposed to feel like.
And, I mean, those effects: