You can figure that no manufacturer is going to sell a stem that's likely to fail, and thus be the source of an expensive lawsuit.
In theory you are correct but companies that make ultra light parts for racing have a different application than everyday utilitarian parts, even those made of alloy. Point being, just cause it's light and thin and expensive doesn't mean you can bash it for years and expect it to be safe. I am talking super weight weenie Ti parts or even aluminum bars and such.