With conspiracy-type documentaries, the narrative is often cleverly devised so that it seems to build on itself throughout the film, making a certain agenda seem more plausible than it actually is. This gives the impression of multiple pieces of information compounding to create a convincing case. Just look at how many people think "Ancient Aliens" is a legitimate factual documentary - people are very easily taken in by the narrative, even though there is basically no evidence whatsoever.
Also, documentaries like this are inherently more biased than any other, because the makers have nothing to lose and everything to gain: A film about a 9/11 cover-up doesn't need to worry about any academic criticism, because they are already telling an alternative story. This gives the director carte blanche to push as shocking/misleading an agenda as possible, subsequently getting more youtube hits, and more money.
It's almost like a "meta-conspiracy"
Yes, youtube seems to somehow be a fantastically good collaborative assist to conspiracy theories.
Check out the "NASA didn't go to the Moon" stuff. It's incredibly bad and devoid of critical thinking or reasoning. Yet many of these videos have 1M+ views. That's not quite Miley Cyrus view levels (800M) but it's very high. Yet high school math and physics will easily debunk these.
What I see is typically arguments based on mis stated premises. For example, assume that the Tower beams had to be melted for the Tower to fall, then proceed to show how jet fuel couldn't have done that. Anyone who's used a torch to heat up and bend rebar would just shake their heads at this kind of idiotic logic.
But these guys just go on and on about it.