Powerlifting as a sport has a set of rules that are designed to make the competition as ‘fair’ as possible. Its also has had a zillion revisions by a zillion federations, which makes it all the more compromised. As they say, a horse designed by a committee is a camel (ie, when everyone has input into the recipe, its bad soup).
For 99% of people, your long term health is more important than the 1% you miss from 'being highly efficient'
Our definition of lower under control
Its a huge disservice to yourself as a athlete to lose a large percentage of your training effect, just to get the most weight on the bar.
Doing controlled negatives on deadlifts, squats and bench is annoying (and honestly, very very hard to learn after years of trying to maximise elastic effect).
Time under tension has a absolute correlation to the amount of stress you accumulate during a workout, and consequently, the amount you can lift.
If you don't believe me, next squat session, make all the negatives 3 seconds and see how much lower weight you have in the top set.
Lowering a barbell in a deadlift is a technical skill - you need to maintain somewhat of a vertical shin angle and get your bum backward, while keeping the bar to your body. Once you clear the knee, your shin angle is whatever is comfortable. In all the years in the sport, very very few people can negative as much as they can lift, so you will have to actively work on this.
The bonus, you get bigger muscles (time under tension) and you don't destroy equipment. Double bonus, negative work has show correlation with ligament strength.
Fix - a two second negative
The Idea that parallel is a magic mark for squats
Hips below knees is just a made up arbitrary point. Broadly, everyone can squat full depth, but the longer your levers, the more mobility, stability and time you will take.
Our default advice to beginners is always to squat as deep as you can, and wear squat shoes with heels while you work on mobility.
If you aren't competing, and don't want to squat to depth, its still 1000000000% better than not squatting. If you are cutting depth, we would recommend squatting to a box or bench so its at least consistent, squatting higher and higher each set makes no progress.
In the larger scheme of things, squatting to parallel has caused a lot of injuries via two things
1. squatter goes a little deep, gets pinned
2. squatter goes to depth, tries to reverse momentum, then gets dumped forward
Given the above, we rarely tell people that aren't competing to max out on squats, and we normally suggest to high bar as your main movement in the first few years of training. We always suggest some level of full depth (high bar of split squat). We know lots of people that literally get CRUSHED if they are two inches deeper than parallel, because they have never trained in that range.
Fix - Consider a full depth squat for your high bars at least, and keep low bar as a comp lift to parallel.
The idea that bilateral is the best (and bar is best)
There was a time in life if you said split squats with dumbbells were superior to squats, I want to fight you. I was quite a dramatic fan boy of squats.
Hate to say it, but for most people, for all the reasons to train (other than absolute strength) a split squat offers equal hypertrophy, superior mobility and a higher level of functional strength.
You don't have to choose, but thinking 'I bench, I don't need to dumbbell press' is wrong.
Fix - everyone should do accessories, try unilateral options where you can.