I've never met a defensive coordinator who didn't fear a spread offense piloted by a mobile quarterback.
There are so many ways that kind of offense can attack. A defense is forced to make one-on-one plays in space. A missed tackle often turns into a big gainer.
I remember a defensive coordinator telling me he always would prefer to play in a phone booth, a colorful of way saying a traditional, pro-style offense is easier to defend.
The downside of a spread offense with a mobile quarterback is the way it exposes quarterbacks to punishment.
Not only do double-threat quarterbacks take hits while throwing from the pocket, they are expected to carry the ball on designed running plays like running backs.