Civilization V

You need to stack. If you're playing with a human or humans, the new combat works (save the navy) but if you are playing with just the AI, war isn't a threat, the AI can't think enough to be a threat with the new combat mechanics.

The AI was more of a military threat when it could stack...

... but then this game heavily punishes you for empire building and that kills most of the fun. Cultural victories and the Space Race always used to feel like a participant trophy that you won because you couldn't get in other victories in time.

Part of the fun was watching your empire spread across the map... and they took that away to focus on more mundane strategies.

The lack of religion and civics hurt too.

But it is lack of stacking and the AIs inability to be formidable with the new mechanics that really killed it

This was the Civ that made me stop buying new Civs... and from what I understand the AI still can't fight a decent war.

So I still play IV, just so being invaded was a threat, because managing culture and tech has always been last resorts to beat the clock or so easy you're ashamed to win that way (and often deactivated to avoid accidental victories).

It just killed the franchise for me.