My shopping panniers are Altura Orkneys (the massive ones with the flap closure, which turn out not to play well with the suspension on the Streetmachine), and they can get quite droopy when there's just a D-lock or similar relatively small dense object in them. As I mostly use them on a 700c upright, it's only a problem if I try to use the trailer at the same time.
Here's barakta modelling them on her 20" wheeled ICE trike with a neatly packed camping load, and there's not a lot of room left for droop (particularly as the rack moves relative to the wheel with the suspension travel on this model):
Your wheels and luggage may vary, but I think you're right about the larger radical bags. (I have vague memories of Auntie Helen dabbling with them at one point?)