A mirror only shows the front of you. If you stop looking into a mirror, you'll find that your ability to "feel" your form through proprioception becomes significantly stronger. And it will tell you things that you simply cannot view in a mirror. Try doing an entire session without using mirrors; if you find it difficult or impossible to maintain proper form, that should be an extremely compelling sign that the mirror has become a crutch, and not a very good one.
There's also the unfortunate fact that, by looking into a mirror, you're guaranteeing that your head is tilted farther back than it ought to be for exercises like squats. You never want to have your neck extended while carrying a significant load. It should be in a neutral position — for a squat position, this will equate to looking at the floor roughly five or six feet in front of you.
Just to add that 'safely' in this context means that if you fail at a rep, the cage will catch the bar. Obviously it can't do anything to point out bad form as a spotter, or even a mirror, could.
I use the squat cage at the gym I go to and I wish they had more mirrors. A pair of angled ones so I can see a side-on view would be really helpful.