I wouldn't say mundane/trivial as much as well trodden. I get good code for basic shaders, various compsci algorithms, common straightforward sql queries, etc. If you're asking for it to edit 500 line functions and handle memory management in a language that isn't in the top20 of the TIOBE index you're going to have a bad time.

The todo comments can be prompted against, just tell it to always include complete runnable code as its output will executed in a sandbox without prior verification.

Fyi, I've never encountered what you're describing, whether with GPT 3.5 or 4.

It may be that you're expecting it to do too much at once. Try giving smaller requests.