Silage is fine until it isn't. I give mine the odd handful as a treat, but I can check the bit I give them for any mold or smell. Free choice like that there is a risk. You just need a bit of soil or a foreign body in it when made and it can become dangerous to horses.
My neighbour (farmer) corrected me when I moved in that baled silage was fine for horses as he'd been feeding it to his broodmares for years. Last winter he nearly lost one of them to toxic laminitis from a contaminated bale.