draft obviously is not a exact science, but its law of averages and probabilities (no, not a math problem). there is enough points of information about players that they can make a educated guess as to which players are better than others, specially in the first round. no, I am not saying all of first rounders succeed, but more first rounders succeed than second rounders. more top of the first half of first rounders succeed, than bottom half and so on. as you get lower in the rounds, the probability of missing information is higher. for example, they played in a particular system, can they make transition to NFL? its a guess. they played in a small school. can they play in the NFL. its a guess...and sometimes players play in a specific system, not similar to NFL, where players do have the NFL skills, but haven't been able to show case it, so they fall to lower rounds because there is less information.
generally each draft has around 16-20 players that are truly first rounders and most go in top of first round and you will find most of them succeed (and yes there are failures there too, but to lesser extent). the rest becomes more of a gamble, that's why you see a lot, a lot more first rounders at the bottom of the round fail. they all have flaws...
some positions are harder to project than others...RB is easiest. OL maybe easier as well, but WR, DEs are a little harder. and of course, QB is probably the hardest, that's why QB success is almost all over the place, although there are years/times that some really good candidates with high probability of success come out, like Lawrence, Aikman, etc.
you probably see a lot of first round QB failures, given current NFL. its a premium position and teams tend to over draft and take gambles....if not for that, most of these QBs would go in lower rounds. ala Lynch.....Geno Smith, etc......
so is it a surprise Dak fell? no. based on then information and analysis he wasn't NFL ready, but he learned, the system he played in wasn't NFL like, but he adjusted quickly. he minimized his mistakes and then he improved his game.