I'd love to give props to Naughty Dog about their AI system. Is it perfect? No. I'm sure most of us have run into a glitch or two - but I can forgive that when the AI is as ambitious and forward thinking as this game
Firstly, the blend of stealth and action. No other game has executed it this well. Naughty Dog has cleverly disguised the typical alert and search patterns of a stealth game right into the voices and body language of the enemies, no longer do we need a gigantic timer telling us when the alert stage will be over - the enemies lose sight of you, and they start searching. They execute smart searching patterns that pressure the player into choices but are also predictable enough that they allow smart strategy to be employed, they are manipulatable as any stealth game enemies should be (with distractions and such) without it feeling unnatural.
When the alert stage is activated (Through being seen or engaging in action) the enemies don't just create pressure and a feeling of a real gun-fight, they also react as you would expect humans to. And that is where the game shines. The combination of AI and animation made it always seem as though I was fighting real enemies, and every choice I made had consequences. The enemies usually made the right choice in terms of taking cover, moving up and attacking. Braver in numbers, and when numbers dwindled against their favour, you could feel the tension shift and the enemies become afraid and emotional - sometimes retreating. In combination with a really visceral shooting and combat experience, I had some of the most intense shootouts in any game
The AI of the cordyceps deserve props for their design, more than their dynamic nature. They were extremely predictable, but this created very fun gameplay scenarios where the player was forced to think tactically. For instance, utilising silent weaponry specifically on runners who can see you and cause an alert, while choosing to use distractions to get around clickers. A really well thought out dynamic there that gave the cordycep's sections their own gameplay flavour
Overall, Naughty Dog did a superb job. As with any AI, there seems to be bugs. But its clearly an ambitious and really thoughtful AI system that the Dogs deserve props for attempting