What annoys me with civ 6 (which relates to the AI waging war) is not necessarily that it's always bad at warfare in general, but that walls and city combat strength scaling in particular punish the civ 6 AI way too much.
As long as a city has walls, the AI doesn't seem capable of handling a well defended city.
The AI is pretty good at rushing down a city (it has enough units and will generally mass them close before DOW), but once walls (and a decently scaling city combat strength from a high tech unit) comes online, the AI can waste nearly its entire army through mindless suiciding right into the walls.
The end result is that I tend to never see any massive runaway domination civs like in civ 5, as the AI generally only manages to take a city or two (mostly those who have no walls, or where the target was far behind in unit tech, causing low city strength) before their advance stops.
Worse still, it can quickly lose the occupied city to loyalty pressure, negating any gains made and as such making a runaway domination AI very unlikely.
This is an issue I have with walls in general though, as once the game hits about the medieval era, even a human advance is very significantly slowed through walls and high city strength.
This lasts usually until the industrial age and flight, where I as a human player have balloons and later bombers to start blitzing again.
As a human player, I know full well how impregnable some cities are when all you have are a few catapults/bombards and a handful of units, and taking a well defended city can often take a lot of setup (moving in multiple catapults at a time for instance).
Sometimes I realize that I just have to give up on attacking the target alltogether because I won't break the city given my current tech level, causing a stalemate.
The AI doesn't realize this and keeps wasting his army, even though its handling of units is otherwise ok.
Anyhow, since the AI is neither adept at choosing to attack undefended cities (and target cities generally having walls after a point) nor using balloons/bombers later on, it hurts the AI disproportionally.
Personally I think the AI would be much more of a threat if walls were toned down significantly.
I want to see a runaway domination AI, but as of now I can't really recall the last time I saw one in civ 6.
The only runaway civs I usually see are peaceful science/culture runaway AIs, which is becoming boring.