It is the worst AI I've seen in a Civ game--heck, maybe even in a strategy game as far back as I can remember. The AI constantly shuffles its (un-upgraded) units when attacking and can barely destroy a single unit, let alone take a city. The AI does sometimes manage to take opponent or City State cities during the early period where cities lack their own defenses, but then they are also likely to trade the city back for peace. After multiple games I have yet to see the AI take a city after the classical era, and have only seen 2-3 wars.
Diplomacy is also completely broken. In my last (deity) game Pedro threw about 10 AT crew + 5 Catapults (??) at my 2 crossbows and 1 encampment/city defensive line. They lost 5 units without destroying one (because of constant shuffling) and then offered their best city, 2500 gold (their treasury), 75 gold per turn (all GPT they had), all of their luxury resources, and one great work of art, for peace. When I took their capital, they offered 4 of their last 5 cities for peace even though I only had about 3 units left.
When it comes to other victory types, the AI seems just as brain dead, but you don't have to be as proactive with other victory types to win (e.g., I've had deity AI win cultural victories just from normal tourism generation).