I trained alone and I never have a problem with squatting alone; without power rack. Just the normal squat stand without any safety. So, how safe is that? Depends. First you need to know the technique to bail out on a fail squat. Once you know, that you can squat to fail without risking any injuries. Second, you need to know your limitation. Train to failure is a big NO for me. Third, I do box squats.