My experience is a pannier does not work very well in rough terrain, that is, where I often ride on my gravel cycling escapades. No matter how well the pannier is attached to the rack, the vibrations make the pannier shake violently, making riding a torture. Once, an Ortlieb pannier detached from the rear rack I fit to a Giant Trance E+ e-MTB. It happened on a rapid descent on a rocky mountain road. The vertical acceleration was as high the pannier "thought" it was being manually detached by the rider, so it detached itself. An expensive camera inside the pannier got severely damaged.
Non-Ortlieb panniers (such as Specialized Tailwind) might be not as fancy or as well designed, but they do lock to the pannier with no chance to detach themselves from the rack.
A good solution has been found by Tailfin with its Alloy Rack. Tailfin even makes a compatible Top Bag, which I perceive as a proper thing for riding in a rough terrain. Tailfin is deadly expensive and of limited availability though.
My plan for the two gravel races I'm taking part in is:
- Sudovia Gravel: Premium gravel roads, many steep inclines. That would be a Vado 6.0 with its sturdy rack and an Ortlieb E-Mate pannier for the spare battery, tools, extra water and snacks.
- Mazovian Gravel: Many nasty singletracks. That would by my Vado SL, and probably only a backpack. I would need to think twice or thrice to risk taking a pannier on that ride. The race will require carrying the bike in several places, so I need to keep the Vado SL really lightweight!
Still, the Ortlieb rack is tempting, also because of the price!