I do think it has gotten harder to evaluate college QB prospects. So many of the good college teams have moved away from the traditional pro-style offense or just have a running QB. With a spread offense, the QB's job is just different and often you're not asked to make the same types of throws that are necessary in the NFL. Someone like Robert Griffin was able to succeed without being forced to throw into tight windows or move through multiple progressions. You see with these college spread offense QBs, even the talented guys, that they often can't adjust to making pinpoint throws outside the numbers.
And, there's a long tradition of these guys failing in the NFL, Akili Smith, Cade McNown, Tim Tebow, Kevin Kolb, etc. The guys that have succeeded from that style of offense, like Alex Smith, seem to have difficulty playing at the level of elite QBs who learned a pro-style offense.
The truth is that NFL teams go through a process of forgetting what QBs have succeeded in the NFL. We have the brief blips where Michael Vick or RGIII supposedly "revolutionize" the QB position and then flame out. Running is a great strategy in college because someone like Cam Newton was that much more exceptional playing in the diluted talent pool of college football. However, in the NFL, those running QBs can't rip off big gains and they end up getting hurt to the point where they derail a few seasons. But, the bigger issue is that running QBs and system guys never develop the elite throwing skills because they have a crutch. They don't need to look at their third progression, they can just run the ball or throw to a wide open receiver in the spread offense.
So, that's the issue I think, the elite colleges have moved away from the pro-style offense, thus making the QB position harder to scout.
This pretty much sums up a player like Jordan Lynch. In 2013 he throws for almost 3000 yards and runs for almost 2000 yards and on the way sets 4 NCAA quarterback records (all for rushing) and was 3rd in the Heisman voting after being 7th for the Heisman in 2012.
But his style doesn't fit the NFL and everyone knows it. He goes undrafted then tries out as a running back for the Bears. Even with a position switch, he can't make the team. Great athlete, but the NFL skill requirements are so narrow and high that there's no place for him at the pro level in his sport.