I agree lol, you definitely want to have bumper plates when doing OH squats. Russ, is right, the OH squat is far, far superior than a hack squat. While it may not be a mass builder, it certainly will strengthen the shoulders.
One thing a coach taught me that really helped was to NOT focus on my knees going forward, they'll do that automatically. Focus on dropping your hips back. Make sure your bodyweight is primarily on your heels.
If the bar is going forward, you need to make sure the bar, when you start, is over your ears, or even a little behind.