As with all good systematic approaches, Lean and Six Sigma fall in the same category, the V-model is great. It is also, basically, just codified common sense, and that codification is key and incredible important. And as with Lean and Six Sigma, people can apply the substance of it without rigidly following the form (read: checking boxes in process description) and be fine. Or they can can follow the form without the substance and be fine on paper and still produce crap systems engineering. Kind of like the old the saying: Inspection ready troops don't pass combat, combat ready troops don't pass inspection.