I'm referring to the aspects of the investigation that have been reported on or spoken about to the media by those interviewed by Mueller. We know many of the persons he has interviewed, a number of them have told the press about the content of those interviews, and it's been easy to draw conclusions about his investigative tactics. CNN and MSNBC do little else than report on their interpretation of his tactics (prosecuting Manafort to intimidate him into providing evidence on Trump, if he has any, etc.). I'm not sure what you're are grinding on about this time, since I'm not criticizing Mueller's techniques.
The assessment that he has a weak file is my own (joined in by many other commentators) based on the absence of any indictments relating to Trump campaign collusion with Russians (really, the absence of any meaningful indictments at all, to this point), and the absence of any disclosure to the public by Mueller, witnesses he's interviewed, Congress, or any congressional witness, of any evidence suggesting that such indictments will be forthcoming. Instead, it now looks like he needs to try to make a case, if there is one to be made, out of interviewing Trump - and that's not going to happen.