I have the extra pockets for my panniers and have to say I wonder how secure they are being just held on with some tabs being under tension on the separate mountings which are bolted to the panniers - the weight of a full bidon would possibly challenge the fittings.
I only put lightweight items in the pockets - usually just a light rain jacket and snacks. Having said all that I have yet to have a pocket fall off.
I regularly carry my puncture kit (2x spare tubes, dense box of spare parts, CO2 inflator etc.) + seat cover and assorted things I don't want to leak on anything (sunblock, insect repellent, chain lube, shewee, etc) in these when touring. When I go for a day ride from the campsite, the obvious approach is to stick the front panniers on the rear rack, and the easiest way to include the puncture kit is to detach the pocket. It's surprisingly hard to do when the pocket is properly full, preventing the bar with the tabs from bending. Re-fitting is quite fiddly.
I've never had them come undone when comedy off-roading or similar. Realistically, I think the most likely way the pocket is going to come off by accident is if it gets snagged on something and a sideways force applied to the tabs eg. when parking the bike (assuming the usual upright configuration with the pockets pointing backwards for heel clearance) or carrying the pannier by hand.
