While the spread offense can pile up a lot of passing yards, it also relies on a mobile QB who can run the ball from the option as well, mostly from the shotgun. The offense depends a great deal on option and misdirection runs. The zone read is used a lot as well, even more so if the team employing the offense has an athletic quarterback who can run as well as pass.
Riley Nelson is more suited for the spread than Jake Heaps. Heaps is more suited to the traditional BYU offense which employs the drop back passer and multiple running backs taking handoffs and not pitches from the QB. Even BYU will run the option play on rare occasions, but it certainly isn't a staple of the offense.