QB is easily the hardest position to scout if you are scouting for the NFL. But it is much easier to scout from the college perspective since you can isolate and capitalize on what a QB does well with a college scheme. Meanwhile, I think the reverse is probably true for DL.dunno - with all the QB camps, passing camps and 7v7 things going on, you can get a better feel for QBs these days. Not saying the skill are common and easier to find, it's just there are more opportunities to see what a kid can really do and evaluate them.
DL on the other hand, just don't have that many opportunities to see what they can do against high level competition. Seeing a guy dominate in HS isn't always a good indicator, and there are not nearly as many camps out there to see what a DL can do against top rated OLs out there. So the skills might not be quite as hard to find as QB skills, but there just are not as many chance to see if a kids really has those skills or not.