PPG isn't nearly the indicator of a good offense or a good defense as points per drive. And net points per drive (NPPD), a statistic that takes into account yards per drive and turnovers is the best.
An example. NPPD shows that BYU lost the game to Memphis largely because of the offense even though the D gave up 55 points, because there were a large number of drives (20) and our offense continually put the defense in a bad place by giving Memphis good field position.