NFL teams don't have to spend 89% of their salary cap. The salary cap floor does not apply to any single season, only to the entire 4 year period. That means that not being in compliance in any single year is not going to be a problem for any team. Only if a team has so chronically underspent that they managed to still be below 89% over the entire four-year period, will they be held accountable.
Even so, there's no additional punishment beyond "spend the extra money." If the team hasn't spent 89% of the salary cap over that period, they'll have to fork over the difference to the NFLPA. That's not ideal, of course, but it's not exactly a huge incentive to just overspend, either.