There seems to be an acceptance that the Smith machine is safer when you're working out alone which I can't necessarily agree with. If you fail on a normal bench you can at least tip one side of the bar down and wriggle out from underneath it. If you fail on a Smith and don't manage to rack it off before it's pressing down on you then there's nowhere to go except try to work your torso out from beneath the weight.
The alternative is to flip the supports into position but that often means that the bar hits them before it hits your chest and you end up doing partial reps to avoid hitting them. Obviously, it helps to know how much weight you can rest on your chest without crushing yourself.
I've had times when I've been lying in the gym on the Smith with 80 or 90Kg resting on my chest while I waited either for someone to notice on the internal cameras or for me to get my strength back. I've even had one time when I was doing close grip and my triceps just gave out and the weight came crashing down and I didn't get the chance to rack it off. I once walked into the weight room to see someone in the process of trying to work himself out from beneath 130Kg on the Smith. Maybe there's just something in the local water which affects our judgement.
The Smith has its risks like anything else in the gym. I wouldn't recommend anyone to use it if they have access to a free bench, especially if they have one with the cop-out hooks near the bottom. If there's no free bench available then the Smith is a decent alternative. There are even some variants which can't really be done on a free bench like explosively thrusting the bar upwards and releasing it at the top.