Your feels don't replace actual numbers.
In 2016 the Cowboys were 5th in scoring defense. People point to that as proof it was a really good defense. But when you dig further you find:
- They were 14th in yards per game
- They were 27th in interceptions
- They were 16th in total turnovers forced
- They were 16th in TD passes allowed
- They were 31st in completion percentage allowed
- They were 24th in opposing QB rating
- They were 13th in sacks
- They were 23rd in yards allowed per drive
- They were 12th in points allowed per drive
- They were 20th in turnovers per drive
- They were 28th in plays per drive
- They were 14th in DVOA
They were good at stopping the run that's about it.
For comparison, in 2016, the Broncos were the 4th best scoring defense, similar to the Cowboys.
- They were 4th in yards per game
- They were 12th in interceptions
- They were 7th in total turnovers forced
- They were 1st in TD passes allowed
- They were 1st in completion percentage allowed
- They were 1st in opposing QB rating
- They were 3rd in sacks
- They were 3rd in yards allowed per drive
- They were 3rd in points allowed per drive
- They were 12th in turnovers per drive
- They were 16th in plays per drive
- They were 1st in DVOA
The point being when you looked beyond points per game, the majority of other stats showed the Broncos defense was very good. When you looked at the Cowboys, you realized that their low points per game total was an outlier. And when you dug further you found that the main reason they kept their points allowed per game down was because they were on the field less than most of the other statistically top defenses.
If you want to criticize Dak, have at it. He's open to criticism. But claiming he failed some really good defensive teams is pretty humorous.