I have heard about sweating in a muzzle loader causing the weapon to be ineffective at the moment of truth but in my experience I have not had this happen to me.
If we have the sweating issue going on in all the muzzle loaders I'm wondering if it is due to the percussion cap not on the cone or insufficient sealing at the bullet because I rational this question to the modern cartridge that gets subject to the same elements as a muzzle loader & yet does not have this sweating issue that I hear so much about.
My thoughts as to why I've never experienced this issue is because the fron of the chamber as well as the percussion cap areas are sealed from the elements & since no new air & moisture can get into the loaded charge it has nothing in there to cause the collection of moisture, similar to a cartridge arm where essentialy they are sealed from the elements & Like mykeal mentioned "ther have been 150 year old ordanance, rifles & revolvers dug up still ready to go off."
Now if those of us that take our side lock rifles out for hunting but not shot it that day & like here in Virginia need to take the primer off to make the weapon inert should think about a simple rubber cap for vacuum lines to place back over the cone to prevent moisture from going into that area, or just shoot the load off & clean for the next day.
I've been doing that for years including in -5*F days & still taken it inside my +68*F home & still not had an issue.