In my personal experience and opinion it's safe to reheat mushroom risotto.
The problem that comes from reheating the mushroom risotto or any other dishes with mushrooms is that bacteria is able to grow on the mushrooms etc when you're refrigerating and then reheating the mushrooms.
It's not advised by health professionals to reheat mushroom risotto but I personally do it myself and have never gotten sick from it.
Just refrigerate any leftover mushroom risotto and then reheat it when needed.
If you're gonna keep the mushroom risotto longer than a few days then freeze it and then thaw it out and reheat it when needed.