It depends on how you design your software and where the API surfaces are and how isolated the pieces can be made to be. Which, if you're writing unit tests at

It depends on how you design your software and where the API surfaces are and how isolated the pieces can be made to be. Which, if you're writing unit tests at the same time as the unit under test, leads naturally to pro-isolation, pro-modular designs, which are both easier to test, more reliable and generally have a more concise purpose.