We took 2 Transalps 30,000kms from England to India including rough jeep tracks etc. We kitted the bikes out with hard panniers before the trip but by the time we got to Pakistan we were sick of them.
They were too rigid so when you crash the force of the impact is directly transferred to the latches that attach the panniers to the racks. This puts alot of stress on the latch & the rack and, in our case, repeatedly broke the latches. Also if you get bumped by something (as happens in the physical contact driving of Asia) the force is directly transferred to the bike and you get pushed over. With soft panniers this is better.
We went to the local market & bought 4 buffalo hides, marked out a design on the back and employed a cobbler from a street-side stall to cut & stitch up the new bags. These leather panniers lasted about 20,000kms and we're taking them to S.America in January. With a plastic liner they are waterproof and they are nearly as secure as our Hepko & Becker boxes that could be opened with a lolli-pop stick. They have also survived 80km/h crashes with nothing more than a graze.
The metal panniers made good packing cases to send souvenirs home in.