I've used magnetic mounts for guns for many years with no issues.
I don't know if they've become slightly magnetized or not because I keep my guns out of piles of metal shavings.....
On a more serious note they are great for keeping a readily accessible weapon close by and mounted in a place almost no one would ever think to look.
Out of curiousity I took a gun that has been on a magnetic mount for years and checked it with a tiny 3/8 x 4 wood screw. It didn't hold or attract the screw at all.