I went to two of the matches in Memphis. The first was Magnus L. v. AK. The lineman sitting in front of me at the start chatted with me for a while. He said some pretty negative things about JM, though I can't say they were unfair negative things. Funny thing was that he was the chair umpire for the next match, Mcenroe v. Courier.
In that match both players complained about calls bitterly, McEnroe much worse, and both players offered to pay the linesperson's wages, and then call their own lines. At one point, McEnroe hit an ace, called out, not overruled, and finally Courier said the ball was inside the line and gave the point to McEnroe. I will say that if I was in McEnoe's position, I would have been very upset about some of the calls. At one point, he said something to the effect that they were intentionally screwing him, which probably wasn't true, but some of the calls were missed badly enough to make one wonder.
Finally, all the players signed autographs for my son and the whole event was fun and exciting.