Surprised no-one’s mentioned the Tailfin rack/bags yet. Not panniers exactly, but same sort of idea.
Was just discussing this morning actually how on the face of it they solve a lot of the problems with panniers for off-road touring (durability, weight, width) while keeping most of the advantages (stability, easy access, attachment/detachment).
As Bob says, small seat packs are fine. The problem is when you try and use a massive one to capacity. But it works for some.