Thing is, with the hard mount, you've got to have a pouch to store the loose extra rounds. That's extra movements - which is crucial time. Imagine doing a load from a bag for each round, or as many as you can hold and manipulate under pressure in a reload situation.
Another nice thing about the esstac type solutions, is that they will fit in standard AR magazine pouches so you could have one staged on a battle belt, or on a light plate carrier. A pouch of loose shells will just swing, make noise, and could be in the way, or even out of reach when you want to keep eyes on target
Slap one esstac on for a ready to go 6-7 rounds, or manually load 6 or 7 each to the hard points. IMO better efficiency with the esstac. But thats just me
As for the attachment method? Clean the surface of the gun with alcohol prep pad or similar, trim the velcro to fit, including the trigger housing pin to access, and attach the velcro. It stays put. Wont mar the finish of the shotgun, even the cerakote titanium ones.
And if the velcro goes bad, easy peel and reinstall with a new pad.
Hope that's helpful