The single biggest issue I had with civ V was the AI. When it comes to diplomacy, they will immediately oppose the player once said player pulls ahead, torpedoing their own economy by refusing to trade, or choosing ideology that opposes the player even if said ideology conflicts with the victory condition said AI is going for.
And the military actions, good lord. I developed a theory, and stand by it, that when Fireaxis removed the stacking system, they never bothered changing the AI to accommodate it. The AI will do incredibly dumb things, like marching armies single file towards a city, or move the artillery before infantry. If I can defend a city against a giant army using just the city and a single ranged unit, your military AI is severely screwed. It makes the game boring later, since you can, quite literally, use two infantry units and a single artillery to wipe out entire nations do dumb to understand how to concentrate fire.
I played a bit of Civ VI, and saw that the military was still braindead, and the diplomats self sabotaging. Its the main reason I never bought VI.
I wish they'd do a proper HD remaster (NOT remake) of alpha centauri.