I've found it to be terrible when you allow it to be creative. Constrain it, and it does much better.

Have you tried the planning mode? Ask it to review the codebase and identify defects, but don't let it make any changes until you've discussed each one or each category and planned out what to do to correct them. I've had it refactor code perfectly, but only when given examples of exactly what you want it to do, or given clear direction on what to do (or not to do).