My biggest gripe with the Civilization series is the fact that the AI will NOT respect borders. In Civilization: Revolution on the Nintendo DS and in Civilization III for PC, the AI thinks absolutely nothing of crapping out a city right next to yours... unless their is a Cultural Border... which is dumb. Irritates me to no end. It's MY continent dammit all!
Since Civ V Firaxis haven't really bothered with opponent AI with the series. They don't even make the AI able to understand the game rules, instead they just have the AI ignore how game mechanics work and the underlining rules of the game to give some false illusion of challenge but it all falls apart very easily because the AI isn't aware how anything actually works.
Take for example things like unit maintenance, the AI ignores it completely and has the ability to keep spawning units as it decides it wants running their empires at extreme economy deficits with zero repercussion because those rules of the game simply don't apply to the AI.
Civ IV had a lot of rule bending with the AI handling, as do most other games. But the level of shoddiness in how they've implemented it in the games since Civ 5 takes it to a whole other level to the point where it's absolutely laughable that a company is shipping something of that low quality and charging £45-£50 for the game (Even 3 years after release) and £20-£30 for expansions that just throw in more mechanics they never bothered to make the AI understand only making the issue even worse.
When they released the 'Rise and Fall' expansion for Civ 6 they introduced a Loyalty system for cities where unloyal cities could rebel and side with another power. Firaxis made no effort to make sure the AI understood Loyalty was a thing, and so the AI would send settlers over to your borders, plonk down a new city that would instantly be in a state of becoming disloyal, then a dozen turns later would rebel and easily become part of your empire... then the AI would keep repeating it again and again with another settler on your or other empire borders. Spawning magical settlers from the ether and then founding distant cities that could never be kept loyal due to distance, just pumping out free cities for their opponents.
The same goes for unit movement, as long as the units are not observable by a human player then the AI is capable of mini-warping units around the place outside of the players vision.... something Firaxis also tried to implement in XCOM and XCOM2 with mixed results. It worked better in XCOM2, but in their first XCOM attempt it led to all sorts of weird bugs and behaviour such as Aliens teleporting into locations you know they couldn't have come from and in some cases the behaviour transporting aliens mid-mission into locations that are right in front of one of your units if they had a tiny blind spot that the system picked up as "Fog of War", even if you'd have other units go through that spot just a few turns back.
Firaxis have simply become lazy in many areas of their games.