The ideal requirements of a defensive back are a compromise within themselves due to the physics of a human body. You need mass and strength to be physical, re-route players, tackle, etcetera, but mass comes at the cost of speed and stamina. You need length to make plays on the ball or jam receivers, but length comes at the cost at the cost of quickness.
That second part is why receivers can often be taller than defensive backs. The problem with humans playing defensive back is we are mostly proportional: we can't easily get tall frames and long arms without having long legs and slow turning hips. For a receiver, who already knows what route he's going to run, some of that quickness can be more easily given up for leverage. By the same token, traditionally safeties had fewer coverage responsibilities than corners, so they could afford to be bigger. In today's game, with so many 4 or 5 receiver sets, the modern safety doesn't have that luxury.
The problem for a school like UCF is they already have to compromise on the attributes of an athlete; speed without size, for instance. Nebraska can potentially land the best possible athlete at those positions. We've already seen in this first class an emphasis on taller DBs. With the aggressive, ball hawking style this staff prefers, that makes a lot of sense.