They work, somewhat, if they are installed correctly. They work with inertia, either mercury, a weight with springs, or lead shot. As the gun recoils, the weight from the three items mentioned above, want to move forward in the opposing direction (actually they sit still as the recoil happens), and counter the recoil.
They may not be good to mount in an inertia locked shotgun, similar to Benelli, and others. The reason why, is these types of actions depend on the recoil to lock the bolt. If the recoil is reduced, there could be a possibility of the bolt unlocking prematurely. Also, never shoot one from a shooting vise that won't allow recoil.