You'd have to be a ****ing moron to injure yourself on the Smith machine
I'm 46 and I bench around 300 lb. I would never try a heavy bench on a smith. I tried with about half that once and my shoulders nearly blew up. I guess it's all relative to what you call "heavy". I've seen people use a smith for squats, with a dime per side.... Which is ok. There's little chance of injury with super light weights, at that level it doesn't matter whether it's a smith machine or a pink kettlebell.