I'm not going to claim to be a Breshad Perriman expert, but I'll say that I watch a lot of American Conference ball and I watched UCF play a lot in '14 and '12 ('13 I didn't watch much as we were in different conferences).
Perriman drops a lot of passes, and he drops passes that will really, really piss you off if you're a fan of his team. He drops easy passes. He can be an electric playmaker, and he is as quick in pads as his 40 time suggests. But he's not a receiver that has the intangibles down. He isn't a good run blocker, his routes aren't crisp. UCF used him to stretch the field, and because of that it allowed him to make quite a few big plays, but he isn't a reliable guy across the middle, and he's not a guy you can rely on to move the chains consistently.
I'm not saying he isn't worthy of playing pro football because I think he is going to make a really good #3 option someday, but his weaknesses make him unworthy of a 1st or 2nd round pick. He's the flavor of the week right now.