Well, it depends on how you define "complexity." LLMs are totally rewriting the notions of what's hard for a computer but easy for a human. LLMs have really bad spatial reasoning. I would have to play around a bit, but I'm very certain you could come up with a prompt where an AI is incapable of properly generating a "simple" app that has some important layout constraints.

And just generally, anything that requires the LLM to understand something LLMs don't understand, it's going to fail. I'd hesitate to give an exact example without trying Astra/Fable but I'm sure they exist.