The point about task variability being normal, and attempts to reduce variability being destructive, is explained exceptionally well in "The Principles of Product Development Flow." Metrics folks want software engineering to have units of work so that it can be measured like an assembly line, or sales, or any other number of things that make sense to quantify. But the reality is that we build and make adjustments to interconnected systems, and that just can't be reduced to arbitrary units. Tasks are heterogeneous and work on different scales. When you try to cram them all into the same sized box, you are actually creating friction and making the problem worse.