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I often work on a giant C++ package, and when looking for problematic code worth refactoring the Cyclomatic Complexity or horrible FanIn/FanOut are usually pretty good red flags to look for. Fixing problems there will usually lead to improvements in the whole codebase.

Of course these numbers can only serve as a hint on what would be worth looking at. Making this some hard threshold after which to fail a build or to refuse a commit would be ridiculous.