No personal experience of the body (only a Hassy back) but I've heard people using the H system in at least -25C/-13F without issues. I've used the back in that temperature, so far no issues. Considering the Credo back I have not heard anything, which probably is good in this case. I know some instances of the Aptus series could have problems, but the Credo is an entirely new design so I would guess that it will work fine.
Almost all electronics and thus cameras are graded to only 0C/32F, but if good quality components are used and the design is good they always work at substantially lower temperatures. The LCD screens tend to get sluggish at -25C, but they work.
Keeping batteries warm is a good idea.
Condensation issues I guess vary a bit depending on climate. Here I can go from inside to outside without adaption (if the temperature drop is really large, than I let it cool down slowly in the bag for safety but I don't really think it's needed). When I go from outside to inside I keep the gear in the closed bag until fairly close to room temperature, I don't need airtight plastic bags, a closed bag is tight enough here.