What it seems like to me is: Fast spread offenses are great if you score. However, they come with two related problems. You score fast, your defense gets on the field quicker. Your failed drives end faster, therefore putting your defense on the field faster. If a team with a defense can stop the spread, the game slowly but surely starts rolling downhill, as the fast quick scoring spread team's defense gets worn out. Its a great offense against teams that have a similar defensive or weaker defense than yourself. Against the best defenses, you are you're own worst enemy.
I think you're on the right track, but the key to successfully running the spread in the SEC, to my mind, isn't just scoring, but rather, it's 3rd down conversions. Explosive scoring plays are nice, but as you point out, they put your defense back on the field. But, string together 10 plays, whether you do it by power running or throwing the ball, and you're not only going to wear down the opposing defense, you're going to give yours a respite.
What makes Manziel so great is his ability to convert on 3rd down. He had less success doing this against Florida and LSU. A&M's 3rd down conversions were what allowed them to win the game against Bama. They had a handful of explosive plays, but moving the chains made the difference.