So many times I get AI generated PRs from juniors where I don't feel comfortable with the code, I wouldn't do it like this myself, but I can't strictly find faults that I can reject the PR with. Usually it's just a massive amount of code being generated which is extremely difficult to review, much harder than it was for the submitter to generate and send it for review.

Then often it blows up in production. Makes me almost want to blanket reject PRs for being too difficult to understand. Hand written code almost has an aversion to complexity, you'd search around for existing examples, libraries, reusable components, or just a simpler idea before building something crazy complex. While with AI you can spit out your first idea quickly no matter how complex or flawed the original concept was.