You bring up some interesting points. The publicizing side of things makes sense. Although given how often you hear about ex-military, police, etc, members in groups like these, I'd still expect that we'd see a handful of criminal/terrorist activities taken out by highly competent groups, even if it's unclear who it is.
But that may get back to your point about discipline, motivation, intelligence, etc.
Re SEAL/Delta-level: I guess I'm asking two distinct questions.
1) To use your basketball analogy: while NBA-level skills may be an unreasonable expectation, why do the videos seem to show middle school or JV-level competence? I'd expect that D3 college-level competence wouldn't be super difficult, but evidently that isn't a correct assumption.
2) What is the practical requirement for a world-class tactical unit (or near that level, e.g. D1 basketball or a bad pro team)? I wouldn't expect current militias to be at that level, but what about less developed nation-states?