Not sure points allowed is a better metric. For one, a bad offense can affect that with turnovers and field position. Secondly, yards allowed is a more foundational measuring stick. Can't score points if you don't gain the yards first. It's like saying "if you score more points than the other team, then you'll win". Obviously that's the case, but you have to look at how those points came about.
There's no real 100% accurate way to measure this though. But just going by the eye test, the Bills defense last year was right at or near the top of the league.