Every NFL team has to take on risk each year. There's no such thing as a perfect roster in an era with salary-cap restraints.
Some concerns are more minor in nature. Like, for example, the Oakland Raiders' depth chart at quarterback should Derek Carr suffer another serious injury.
Others are much more serious, like the Arizona Cardinals being worried about quarterback Carson Palmer fading into dust, or the uncertainty that surrounds Blake Bortles' career with the Jacksonville Jaguars.
If general managers think about it long and hard enough, all 32 can find a way to deprive themselves of sleep.