Ask HN: What are the most complicated types of programs?

Your list is generally what I think of first.

I once inspected a small subset of the software for a real-time system that regulated industrial machinery in a large factory. That stuff was intimidating. I can only imagine the complexity of seemingly simple gadgets that human lives depend on (e.g., power plants, avionics, medical equipment).

I could present evidence that FizzBuzz is too complex for some of the applicants that I have talked to over the last few years.

Multi-country support for corporate tax and employment.

Or even just billing. Like eg subscriptions. Or medical billing.

Web browsers would be up there.

Yes agreed. This may be a stretch, but one could consider web browsers to be an OS, compiler, and graphics engine all in one. Yikes.