The ISS problem is arguably harder. It doesn't need to survive reentry but you also can't take the whole thing apart, clean it, and put it back together again i

The ISS problem is arguably harder. It doesn't need to survive reentry but you also can't take the whole thing apart, clean it, and put it back together again in a nice climate-controlled machine shop. You have to do it in cramped zero-gravity and keep most of the systems online while you're doing it.