Of course ROI and similar metrics are always macroscopic...while "easy" is a case assessment (non-macroscopic), but Herley points out that the case assessment doesn't really matter on the macroscopic level, where it's the rate of success taken over great volumes that takes first priority.
A bit like the difference between molecular physics and chemistry. The potentially erratic behaviors on the molecular physics level aren't important to chemistry, because the chemical behavior is made up of huge numbers of molecules, evening out the unpredictability.