I think strap in pedals causing knee problems is complete nonsense .
I like mine because being able to pull up with one foot at a time tremendously helps with power output. I can spin faster and more efficiently in a lower gear. Without them, I used to always pedal lazily in a super high gear. Its particularly helpful going up hills.
If you have knee problems, I suggest trying to find a local bikehub program and try to find some spare biopace rings. They are so easy on your knees.
Biopace is oriented so that the most powerful part of the pedal stroke (9 and 3 o clock) has a slightly smaller tooth count to help speed up the crank to get you through the weak spot (12 /6 o'clock) faster. I thought it worked tremendously well... until I installed my strap in pedals. Since I can pull on one side while pushing down on the other, the concept of biopace is totally useless. So I went back to circular chainrings and was more or less happy for the time being...
Then, I ended up going back to biopace yet again,.except this time I found a double setup with the perfect ratios for my needs and I made sure to orient the rings backwards so that the power part of the stroke correlates with an increase in teeth count. Still very easy on your knees, less bouncing, better power transfer... its a super natural feeling and it really helps me take advantage of my gearing. I'm always spinning at a higher cadence now even in larger gear ratios.
This,.paired with the right side crank arms, the correct ratio front rings, and a more aggressive rear cassette with closer spread is a real game changer for performance and comfort/efficiency. Id never go back.