* The unit is no longer portable and can't be pulled from the context it was first used in (e.g. into a library or another app) without becoming untested. And adding characterization testing later is usually more expensive * A developer who needs to make a change to that unit needs to know where to "test drive" that change from, which requires that they know where to look for the parent's test that uses it. That's hard enough but it completely falls over when the unit is used in two, three, or more places. Now a bunch of tests have to be redesigned and none of them are easy to find. * Integrated unit tests like this lead to superlinear build duration growth b/c they each get slower as the system gets bigger. This really trips teams up in year 2 or 3 of a system.