I use their oversized safetys on several of my Mossys, and have a couple of their followers. Top notch stuff.
If memory serves, the mag tubes are steel. They market a bayonet adapter kit for shotties that includes the adapter and tube. All of the components are steel I believe.
Besides being heavy, there is a corrosion concern when you put steel and aluminum together.
Nordic makes excellent aluminum tubes, if you want to check them out. Pricey but great quality.