When scouts miss a guy who turns out to be great, it's invariably because they underestimated the guy MENTALLY. Physical ability is easy to measure. Anybody with a laptop can look up a guy's college stats But nobody can seem to measure a guy's ability to learn. Unless they're an egregious moron like Johnny Manziel it's just a black box.
Tebow was lauded for his intangibles more than anybody in modern scouting history, but if you watched him practice in 2015 you saw the exact same thing you saw in 2010. He was literally incapable of learning a single thing about being a pro QB that he didn't already know in college. In strictly football terms he might have been dumber than Vince Young and Michael Vick, which is saying a great deal. RG3 was supposed to be super smart and did nothing but backslide from a rookie season spent just running the Baylor offense again in Washington. Hell, on paper Ryan Fitzpatrick is the smartest man in the NFL and he can't resist throwing to opposing defensive backs after 12 beepin' years!
In a lot of ways Dak was another McNabb coming out of college (who Zierlein forgets was actually very raw coming out of Syracuse.) Both were really sturdy 6'2" guys who had mobility but mainly wanted to pass. Both came from schools that were not traditional football powers. Dak played tougher SEC competition, but McNabb put up similar efficiency numbers in a less pass-happy era and with a worse supporting cast, so that's arguably a wash. Both were seen as good high-character guys with some mechanical issues. Both had good arms but neither had consistently pinpoint accuracy.
The X factor that sets Dak apart is his ability to pick up the NFL game way faster than normal. He's just way ahead of the game from the neck up compared to every other rookie (and many veterans, e.g. Weeden, Cassel, etc.), and that's what the scouts totally wiffed on. McNabb was seen as a fast learner, and he was still an awful rookie.