I don't think steam cleaners are a good idea for Artex as they might bring the whole lot down in a soggy mass. We have had some of our ceilings that were Artexed plastered. We got someone to do the job and it has worked well.
Funny how things change. When we bought our first house in 1972/3 we took down all the old lath and plaster ceilings, put up plasterboard and Artexed.
I am sure that a wall paper steamer would do that. The literature on steam cleaners says that they are putting out far less water and heat and are not wall paper steamers. But like you, I did wonder whether it was safe for Artex as on all the lists of things it is safe to steam, Artex doesn't appear.
If anyone else has any hot tips on cleaning mildew or Artex that would be appreciated. We are just not going to replace or skin the ceiling at the present - limited time and money. We just want to get the ceiling clean and painted as is, in the most effective way possible that doesn't involve removing or covering up the Artex.
Take care - steam cleaners are used to removed Artex. Take care, too, if it's old Artex; the older stuff had asbestos fibres to strengthen it and increase its fire resistance, and you don't want to be breathing those in.
It really is hateful stuff, and when I am dictator it will be banned.