Why is putting hot milk in a thermos not recommended? A colleague of mine just got a new thermos (inox), this model, and actually read all instruction, and we were surprised to read that it is not recommended to pour hot milk in the thermos. 

My opinion was that:


Unlike tea of coffee, a lot of micro-organisms live in milk, will end up being a favorable place for those micro-organisms to generate a culture (which will be more and more difficult to get rid off, and will contaminate any other fluid poured in the thermos)
If you don't wash it straight away, it will really stink.
The milk being full of water-insoluble proteins/fat, it will "coat" the internal thermos walls (gross!) - I've already seeing that "coat" forming in plastic containers, and I guess it won't be any different in inox containers.


I've actually not found an answer online, so I thought that here I could find some experts. I realize this could be a cross-question for Biology SE, but I felt it more food-related.
The cleaning is one thing (there are flasks without hidden crevices that can be easily cleaned), but there’s also another point to consider:

A thermos flask that is doing its job will keep food warm, or rather, slow down the cooling process. Even if you fill it with fairly hot liquid, it will slowly cool. If you do that with a perishable food like milk, you will easily create an environment in the upper range of the danger zone, where food will become unsafe after as little as two hours. 

Yes, you can argue that you are filling hot milk in a clean container, but in the end, it’ll be not safe by food safety standards. (What you make of that is obviously your choice.) The manufacturer’s warning will protect the end users from food poisoning.