Safe … Yes, assuming there´s a fabric behind the board stopping particles from the wool escaping.
Effective … Well, depends on what the purpose is but assuming to limit the frequency range of absorption to the lower part of the spectrum; yes, depending on the thickness and perforation percentage of the board.
I put it to use. I needed a way to hang stuff and I was already going to build a frame for it so why not stuff it with the good old mineral wool. They seem to be all the rage because VKLA has them all over the place.