I don't think that's true. USCIS does the same background check for everybody, one background check isn't harder than another. They check the FBI criminal database just like everybody else.
If you look at the people still waiting, nobody really has any special circumstances. And most are approved.
As for leaving people behind, that's exactly what they are doing to improve their statistics. They leave a large chunk and lower priority to a trickle of what it was (5 to 20 cases a day), and move onto the next month.
It comes down to being unlucky, unfair, incompetent and heartless. There are so many people in 2021 who are still waiting while people almost a year after them are being approved.