The real answer is both.
Meth labs, as already stated use a lot of chemicals. Ether, phosphorus, etc are just some of them. The house is basically turned into a lab which looks like a chem lab from an old movie with glass tubes and chemicals everywhere. There was a meth house here a while ago that was so bad the entire block had to be evacuated and the sewers had to be pumped out.
Pot houses can be just as bad though. The standard operation for a pot house is to rent a place, then at night wheelbarrow in several cubic yards of dirt and cover the inside of the house. Of course, all the windows are shut so no one can see/smell inside, and the humidity and temperature are turned way up. They get as much out of the place as they can before it rots from the inside.
In both cases the houses are completely not usable.
Of course, you could have a small meth lab in a basement, or a few plants growing in a closet and all that's needed is a cleanup of the affected area.