I know little first hand about either the Benelli or the 590, but I've some insight into the military mind.
First, you should recall that the M-16 was never intended to be a battle rifle, but a survival tool that was to be strapped to an survival kit, or liferaft. Once some fool decided it was a line werapon, it took about 5X as much money to make it reliable and effective as it did to develop the concept.
Second, the adoption of that Beretta 9mm in place of the old Colt warhorse has seen more NDs and injuries in 20 years than the GM had in 70. And, the Beretta rep for FTFs put it way down the list of picks for personal defense.
Now to shotguns....
Ever since the Brown Bess was state of the art, military weapons have ben built as ruggedly as possible. The Garand, K-98, AK etc, come to mind.
My guess is the 590 doesn't hold up, and since the folks that picked it don't want to admit that, they moved on to a "New, Improved" shotgun rather than regress to an older, previously used model.
Point of Fact, possibly the best fighting shotgun from a military POV is the old Model 97. Tough, reliable, ambidextrous, with relatively high mag capacity, it's sole drawback is the fairly high cost of machining the parts. The experimental model w/ grip safety done at SA might still be the best choice in a military shotgun.