You can read the related blog post at https://jonloomer.com/placements.
Mistakes related to placements tend to come down to one of two things:
1. Inaction or
2. Too much tinkering
In most cases, you should use Advantage+ Placements on when optimizing for any type of conversion.
When you should remove individual placements is when optimizing for link clicks, landing page views, or ThruPlays when Meta can get you low-quality actions as a result of placement weaknesses.
In this video and blog post, I provide further details, as well as examples for how the algorithm adjusts, depending on your optimization.
