The main coding agent failure modes I've seen:

- Proliferation of utils/helpers when there are already ones defined in the codebase. Particularly a problem for larger codebases

- Tests with bad mocks and bail-outs due to missing things in the agent's runtime environment ("I see that X isn't available, let me just stub around that...")

- Overly defensive off-happy-path handling, returning null or the semantic "empty" response when the correct behavior is to throw an exception that will be properly handled somewhere up the call chain

- Locally optimal design choices with very little "thought" given to ownership or separation of concerns

All of these can pretty quickly turn into a maintainability problem if you aren't keeping a close eye on things. But broadly I agree that line-per-line frontier LLM code is generally better than what humans write and miles better than what a stressed-out human developer with a short deadline usually produces.