I think you’re mostly talking about things that are real, but have had minimal impact (the DCCC backing the wrong horse in a primary), things that are dubious (leaving seats uncontested where Democrats would have had an advantage; how many of those can really exist?), or things that are outright false (Hillary getting a free ride in the primary).

I do give Howard Dean some credit, through the Fifty-State Strategy and small-donor fundraising advances, for facilitating some marginal wins in 2006 and 2008. Competing everywhere is an important prerequisite to unexpected wins in new territory. But when a Democrat is in the White House, that Democrat controls the party, at least until the next nominee is chosen. So the DNC chair was largely irrelevant from 2009 to 2017.