I watched the film (on youtube) and don't like what I see
You watched clips of 3-6 games on youtube, and you know he will be a bust. From someone who has seen 48 of his games, you are wrong. His tape usually consists of the Ohio State game, which he dominated, the Baylor game, which tells you nothing about Mack if you watched the game and not just clips, and the San Diego State game. In the SDSU game, this is what the youtube clips don't show: Buffalo was down 14-10, and SDSU was on the 7, Mack made a huge stop on 3rd down, which was nullified by a penalty, SDSU would later score to go up 21-10. Buffalo threw an interception and fumbled the opening kickoff, and the score was 35-10 the next time Mack hit the field. At that point it looked like, game out of hand, he was worried about keeping himself healthy, he had 20 million reasons not to go all out in a game that seemed lost.
Oh and everyone who mentioned watching tape on Mack, didn't mention the defense. UB often dropped 9 men in coverage and rushed 3 linemen on passing downs. This almost guaranteed Mack would see a triple team on passing downs, and he still put up great numbers.
I haven't seen him play
Many people don't like the Mack pick because he's unknown. This is a complaint I here more from sports talk radio than from fans. It always makes me laugh, UB had 5 ESPN games, and one game on Fox Sports Southwest. The Raiders were not a good team, and Mack was a highly touted prospect coming into 2014. The talk radio hosts are basically telling us they don't do their job very well.
He played bad competition
This is true, however the flip side is also true, he had little support from his defense. Ever since his sophomore year, every offense Mack has played has had a game plan, stop 46 by any means necessary, and we'll deal with the other 10. He may not have faced NFL level tackles, but he never faced just ONE tackle, he had to cut through double and triple teams, and get to the QB before the often weak secondary left a receiver open.
Clowney played the best of the SEC, but he had SEC talent all around him.
Stephon Gilmore 1st round CB
Melvin Ingram 1st round LB
D.J. Swearinger 2nd Round Safety
Devin Taylor 4th round DE
DeVonte Holleman 6th round LB
Antonio Allen 7th round Safety
Travian Roberston 7th round DT
Steven Means 5th round DE
Mack at Buffalo was like a batter warming up bat weights. Put him on a defense where he isn't the target of double and triple teams, and he will be even more explosive.
He is not a sure tackler
This is probably the most astute observation, he comes with speed and trucks, more than wrapping up and form tackling. This is good and bad, some tackles are missed, but most are made, he's a big strong man. The other good is his unorthodox style leads to him seeking the ball on every tackle, which is why he has 16 forced fumbles. The bad can be negated by better support. Mack can do what he does best, make carnage and break up a play and the rest of the Raiders can come in to finish the job.
He is weak in coverage
This was my number one concern coming into 2013, Mack responded with 3 Interceptions.
He needs to develop
He was a fifth year senior, and is twenty three years old. He is also hard working and mature. If he needs to develop, he will, which is something I'm much more sure about than a Manziel or Clowney.
Buffalo who comes from Buffalo?
Former Raider Trevor Scott came from Buffalo, he had 5 sacks in 2008 and 7 in 2012, tied for the team lead in each season.