Something to consider is that runner form in weekday fields is generally far more unreliable/inconsistent than form for a metro Saturday field's runners, which might explain [to a degree] why more surprise long shots get up during the week than they do in metro Saturday meetings.

This weekday "hows your father" form may well have a negative affect on many systems, especially at typical weekday tracks that get far less money spent on them["hows your father" tracks], than Sat. Metro tracks. So there is two possible negatives for weekday meetings for any system and there are probably others, like those mentioned by Angry Pixie.