Although modern gas operated systems are very reliable, there is still the potential for more to 'go wrong' than with Benelli's recoil system. The recoil system has fewer parts (no pistons, etc...) and therefore has less parts that can fail. Also, they are much cleaner...
The only real cost with the Benelli is the fact that they are expensive on the front end (when you buy it) and the felt recoil is more than any other shotgun. The M2 is supposed to be designed to take away some of that recoil, but I have not shot one to give my opinion.
I do have an M1 Field in 12 gauge...I love it. It has been flawlessly reliable and is really easy to disassemble and clean. For hunting it is the perfect auto shotgun IMO...because when you hunt, you don't shoot as much as when you are shooting skeet, for example. So, my shoulder can take it.
Benellis are wonderfully simple shotguns and they work all the time.