A brand of technical bags is judged by its specialty in origin: cycling, hiking or the mountains. Real waterproofing and the fastening system distinguish it more than its name.
Technical bag brands come from cycling, hiking or mountaineering, and their origin is evident in their products. A bike pannier brand focuses on mounting and waterproofing. A hiking brand thinks about carrying and adjusting the back.
The specialty of origin explains the construction choices. Bicycle specialists work on quick mounting and welded fabric. Hiking specialists focus on hip belts and adjustable back lengths. Mountain brands work on equipment access and abrasion resistance.
Not all advertised panniers are waterproof. A coated fabric can withstand a shower, while welded fabric with roll closure can handle short immersions. The difference lies in the seams: welded or just stitched and covered. This is the only point that really separates two brands on this criterion.
A pannier can be attached with hooks, straps, or a proprietary rail. The proprietary rail holds better but locks you into the brand's ecosystem. Check the diameter of the compatible tube before buying: many bags won't fit on an unusually sized bike rack.