Players who do not play much each week tend to lose focus and their "Practice this Week" will be sloppy. Players who play too much in a week will get tired and their "Practice this Week" will also be low-quality.
It is not there skill training that is impacted, but rather their form. I'm not sure how "too much in a week" is known to be 90 minutes, but that is what I've always heard, and try to keep my players under.