[squiggleblaz]

On legacy code? Alas. The amount of changes you might have to make to make the legacy code unit testable versus the benefit of unit tests can be exceedingly significant.

For instance, if the implementation involves calling up a bunch of value objects from a database, each of which do the same, and all of the code is inhouse so there is no standard mock or stub libraries available, adding unit tests is tantamount to rewriting the whole system without tests.

Existing codebases can also be too complicated for a few people to formalise into unit tests. The algorithm itself might be simple, but again, to discover that, you need to rewrite the whole system without tests. (You can add tests, of course, but when you're adding tests, you're making an assertion you cannot prove. Since you don't know what the code is meant to do, you don't know whether the tests are complete or even accurate.)

Once you're coding in a world where tests passing or tests failing has almost no predictive value on success or failure in release, you're working in a self-fulfilling prophecy where the code will never be tested because the tests literally make things worse.

Many codebases clearly do not have any automated tests at all, but unit tests can be the hardest to add onto a system after the fact.