The issue seems to be that she could of hid something in her personal emails by not turning everything over. But couldn't any official have just used their gov email for "official business" and use their personal email for stuff they want to hide? Doesn't every politician actually do this?
The fact that Clinton is letting personal correspondence mix with government correspondence indicates to me she has less to hide, not more.
The only issue that has the possibility of being legitimate in is the Benghazi stuff. Technically, it could have been a deliberate move to make emails related to Benghazi un-searchable during the investigation. Of course, this only actually matters if those emails contain anything significant/damning, and they probably don't.