Wittes' complaint is that Barr misrepresents what Mueller's meticulous investigation found. A more concise version is also available from the New York Times. Both are worth reading. The NYT version has direct comparison of Barr's statements versus what Mueller reported. It's quite scathing, even though the author doesn't call Barr a liar. The author does highlight phrases from Mueller, and shows how Barr distorted them. Barr did it so shamelessly that he comes out looking horrible.
Wittes' takedown is more complete, pointing out precisely the twists and turns in Barr's distortions. For example, Wittes shows how Barr claims there was no collusion, when in fact Mueller found evidence of interactions between Russia and Trump's campaign. How is that nothing? Because there wasn't enough evidence to make charges with confidence of a conviction.
The conservative site Washington Examiner methodically rips Barr apart. Or moreso, it rips apart the idea that Trump should be the judge of whether he is being investigated unfairly:
"No man, not even the president, should be allowed to adjudge his own case... The very notion of such power runs counter to the entire basis of America’s constitutionally limited government... "That is a simple, elegant explanation. Even the president has to allow an investigation to go forth. It's not for him to declare his innocence and shut down the process.
McGahn, the White House counsel during much of the investigation, is quite a contrast to Barr. He would push back against Trump, refused to fire Mueller, gave 30 hours of testimony to Mueller if I remember correctly. He also consulted with his chief of staff, who made written notes of McGahn's meetings with Trump. It sounds as though Trump didn't want notes taken, which hampers clear instructions and the ability to review what's been decided. McGahn has been subpoenaed to testify before Congress. I certainly hope he does. I'd like to hear some of what Mueller's investigators heard, directly from the source. But McGahn may not be willing to discuss private and privileged conversations.
Extras. Reviewing how the Congressional response when Mueller was appointed from three sources (one, two, three). More wrangling over the Mueller report, this time by the lawyers for Roger Stone. They are trying to say "No collusion, so there was no cover-up, and obviously no lying to Congress." But maybe there was lying to Congress, and maybe some cover-up, and perhaps a wisp of collusion.
Update 6/7/19. Another good article showing precisely what the lies are.