stunts/twists are obviously a nice way to mix up the defensive line rush/run defense and add some confusion to the offensive linemens' reads. but stunts/twists are not a static thing...there are variations of them including fixed vs. read stunts. there are significant risks associated with them in both the run game and passing game but definitely moreso in the run game.
in the run game, the biggest risk is losing gap control and putting a lot of pressure on your linebackers. it has the potential to make a well executed inside counter that would go for four yards go for 20 yards, etc... it can also have a big impact on run fits and totally undermine them.
they are generally considered to be significantly more effective in defending/rushing the passing game and rely on poor communication and discipline from the offensive line to create big defensive plays (the type we see from campbell and williams routinely). the biggest risk in the passing game along with the run fits is that the rush lanes can get wildly out of whack and breaking contain becomes especially easy. it's extremely risky against vince young, cam ward, johnny manziel, etc... free ten yards every time because one linebacker can't cover everything. similar risk as an all out blitz that misses and then jalen milroe is running 60 yards down the field by himself.
for the most part, the last two years, PK has had the luxury of having a much better defensive line than the opponent's offensive line straight up and can contain the run game and make the passer uncomfortable while running a base defense. pk is an extremely risk-averse d.c. and if you are just way better than the opponent, then there is no need to roll the dice and increase the risk of giving up an explosive play. pk is pretty consistent that if it is 3rd and 8, he is going to flood the zone and count on his 3-4 rushers creating pressure. sometimes that is very frustrating because some all out blitzes in key moments can wreak havoc but he currently has the dlinemen (and last year) to keep it relatively straight forward and limit the downside potential.