We had a framed backpack but we got rid of it as we found soft structured carriers better. We found it took up most of the boot, and wasn't comfortable. The only real advantage we found was that we could put it on the ground with the baby sitting in it like a high chair, but she soon got too big for that.
Even if the backpack is waterproof and has a cover, the child inside will still be exposed to the elements, so it is a better option to have a fleece all-in-one and a waterproof all-in one so that the child is protected even when out of the carrier.
I do a lot of hillwalking with a soft structured carrier. I use poles for stability on my descent. The carrier is made of cotton, so it is absorbent but that isn't a problem as I always have a waterproof suit for DS. With the children I am a fair weather walker so it is more a case of having the correct equipment to be prepared.
The centre of gravity is lower on a SSC so they tend to be more comfortable over longer periods. For walking on rough ground it is important to be able to balance properly and this can be quite hard if the child is sitting up high in a framed pack. The framed packs usually have some kind of storage compartment that you could put a few essentials in, but SSCs don't have anything larger than a pocket. We get round this by DH carrying a large pack with waterproofs and food for everybody, and me carrying DS in the sling and wearing a bumbag on my front. From a safety point of view this is not ideal as everybody should be carrying their own things but I expect your walks will not be too ambitious with a young child.
I can't recommend the framed packs, and I can't even remember which model we had. If you are serious walkers you will probably find that a SSC lasts longer. My DS is over 3 and I can still walk with him on my back. I wouldn't be able to do that with a framed carrier.
I know this probably isn't the advice you were looking for. Maybe someone who has a different experience with the framed carriers will be able to contradict me.