Lippert sold me their Lippert-designed 1-1/4" receiver hitch that mounts on their Lippert frame, using brackets from Lippert. When I spoke with them, they told me that installing it would void their frame warranty. When I asked why usage of their parts, on their parts, would void their warranty, the answer was that they didn't know how I would use it.
So obviously, Lippert feels very free to say that anything will void their warranty.
With typical goosenecks, as you would see on a ranch, the gooseneck has a long extension that puts a lot of stress on the trailer frame. And on gooseneck trailers, you'll also note matching reinforcement of the trailer frame to deal with that stress.
On fifth wheels, the standard pin has a very short extension that puts much less stress on the trailer frame. The frame design does not include the same level of reinforcement found on a gooseneck frame.
So when you put a gooseneck on a fifth wheel, you're introducing more stress than the frame is designed to handle. The Goosebox, used in conjunction with a gooseneck, is designed to deal with the excess stress so it isn't transferred to the frame.
My understanding of the Andersen hitch is that while in some respects it is like a gooseneck, it has a shorter extension and is believed to present a lower risk to the trailer frame.
That all said, the Andersen hitch is pretty popular, and I don't think we're seeing reports of frame issues. And while you should check with Andersen, I seem to recall they have some type of high-dollar guarantee that would deal with damage caused by their hitch.