LLMs don't make bad tests any less harmful. Nor they write good tests for the stuff people mostly can't write good tests for.

You just end up writing needless tests trying to trigger or mock error state from a 3rd party library that's never actually returning error, just the lib had a rule of "every call returns error code" in case something changes and it's needed.

The problem is that it is natural to have code that is unreachable. Maybe you are trying to defend against potential cases that may be there in the future (e.g., things that are yet implemented), or algorithms written in a general way but are only used in a specific way. 100% test coverage requires removing these, and can hurt future development.

You forgot difficult. How do you test a system call failure? How do you test a system call failure when the first N calls need to pass? Be careful how you answer, some answers technically fall into the "undefined behavior" category (if you are using C or C++).

all of the above.