no probs
We have several ammo storage speed feed stocks in the district, and I have seen them shot a lot on the firing line, and have not witnessed any dumped rounds from the stock. But I suppose it certainly could happen, just not in my experience. But it is a device and devices fail.
I like the idea of on board extra ammo for a duty or combat shotgun, particularly if not equipped with a mag extension. Four, or 5/4+1, if you have time , is not much ammo to go to a fight with.
Onboard ammo increases the weight on any gun, so I think on a hunter, its not recommend. And ducks, turkeys, etc, don't shoot back. One of our guys has a 20" 870 w/ extension (7 shots) side saddle (6?) and speed feed (4). Weighs a ton, but 17 rds of 12ga to grab and run (to cover of course).