well clearly it doesnt start as liquid it has to be melted and put through then boiled. my point is you would have to do the same with a sawyer or other system and then the same question applies, how do you stop your bag or filter freezing. If the millbank freezes it will work next time you pour liquid into it. Your sawyer will be bolaxed
I love my sawyer, thirst aid bag etc but its not the be all and end all but i guess it has come and is largely gone. I would point out i do not own a personal milbank. Mine is the group version which holds around a gallon. It will process a lot of water very quickly
I've never filtered boiled water, but I suppose in that role it would work. Still, seems like a lot of pain for nothing. Carrying around a heavy, frozen piece of fabric just to remove a few pine needles is not going to make it a big seller.
As far as filters, of course they don't get used in freezing conditions for obvious reasons. Under such conditions, liquid water is rarely available though, so it doesn't matter much since you have to melt snow for it anyway.