The little tabs are sufficient only to hold the thing in place while you use a real fastener.
You're nailing into the side of s beam. It is quite possible for an ordinary nail to pass all the way through- meaning there is length you don't use. The fixtures also have little weight, which means you don't need a whole lot of thickness either.
While I prefer to use screws - much easier if you need to re-position the can a bit- I see nothing wrong with using roofing nails.
