I think there's a lot of setup and context required for an AI agent to consistently write good code. Once the agent has these guard rails in place I usually get great quality- far better than what I would write in most cases.
I think where things get dicey is being able to write in any language. I write and review code in many languages and frameworks I'm not fluent in, so it's hard for me to distinguish between working code and great code. I can spot when the fundamental logic is wrong, but when it comes to "best fit" choices I'm clueless.
I'm curious as to what guardrails you've tried.
This is something I have been trying to get right as well. I've attempted to use lots of linting and things like strong typing, duplicate checks, cyclomatic complexity, and robust tests. However, I still happen to find issues, which requires me to look at the code (at least at a high level)
For example, I can say "Don't repeat yourself, and don't re-write helper functions" and I will even have a duplicate linter check, but inevitably the LLM will always want to re-write a similar yet slightly different helper function. Like it will always want to re-write something small like a trim() or a toString() function in every file.
I would say that it is like gardening. If you let them go havoc from the start, the weed will take over. If you keep focusing on removing the weed and enforce specific standards and practices over the code base and it keeps growing, over time LLMs start to suddenly follow that and they don't make so much slop anymore. At least that is my experience. But I force specific audit agent after every added feature which says them to force compliance with AGENTS.md and check the consistency with the code base.
The issue is that in order to have the agent write good code, you need to implement standard SWE best practices. But that also means a lot of manual intervention in terms of writing specs, checking acceptance criteria, and reviewing code. So you end up spending a lot of time on managing your agent, which means you won't get a 1000% productivity gain, you get maybe 50 or 100, possible less in some areas and with some issues.