Assuming that ease of validation and acceptance and confidence in quality = "easier" and more difficult validation and acceptance and lower confidence in quality after refactoring = "harder". Also ease of determining the scope of effects of a proposed change = "easier", then having an explicit interface does mean refactoring is "easier". Refactoring cannot cross an interface boundary without breaking tests, so by definition, you have contained problems to that boundary. If you have no containment boundary, or the boundary of containment has a greater surface area, then it is "harder".