Poor posture, tight hamstrings, weak ankles, knee joints, tendons, etc. You can't take those things for granted.
The more complex the movement, the more likely you are to involve a part of the body that is not capable of handling it. Other parts will compensate, but if you go to failure or use imperfect form, you'll quickly discover a weak link.
Complex movements are the best for maximizing strength gains, but even without weights, they can more easily break you.
Learned this the hard way. I'd encourage anyone tempted to try out his program to take a week or two and work on your mobility first. You won't really notice your imperfect movements until the weight is sufficiently heavy enough to start hurting you.
Interesting; do you have any links for advice or guidance on how to work on mobility?
The weights also improve flexibility but not in as balanced a fashion. Stories abound of lifters with the posture of a gorilla.
Alan Thrall runs a Sacramento gym and has a number of videos on technique: https://www.youtube.com/playlist?list=PLibo-Z1-9Z6jPVfvHhy0v...
2 - Failing (1), find one on the internet that does online coaching. Film yourself and pay for feedback. I have no experience with him, but George Leeman is strong as hell and knows his technique. You could start here. https://www.muscle-massochist.com/services/
3 - If you're in sf, I have a strong recommendation. (Your username may be old.)
Squatting correctly is hard. It's a complex movement that involves the entire body and requires you to do things you probably are not used to doing, and mobility you probably do not have.