I sometimes hear about NFL GMs being "cap experts" rather than player-evaluators. And I just saw this headline: "John Idzik joining Jags as consultant". The article goes on to say, "Idzik, who was fired by the Jets in late December, will not have a role in player evaluation but will work with the team's salary-cap department, the source said" (emphasis mine). Sure the salary cap rules aren't simple, but how do they justify an entire department?
This interview by FiveThirtyEight of Paraag Marathe, the 49ers' cap specialist, states:
We’ve got four or five folks, whether helping scouts better evaluate players, helping coaches, as well as the salary cap.
Basically, the "salary cap department" is code for "analytics". Not only are they working on actual salary cap issues - which is undoubtedly more than one person's full time job, for the same reason as even a small company needs a full time accountant; keeping in compliance with every small detail is important - but they're doing other similar analytic tasks, such as analyzing player performance, finding players who fit their cap space and also are predicted by their statistical models as good candidates, etc.