He wasn't critical of his musical ability. He was critical of him taking jams that he (Ginger) and Clapton started, putting words to them and then taking all the royalties. He also claims that a few of the songs Bruce wrote in complete different time signatures and the songs were pretty well crap, until Ginger rearranged them and they became monsters ( White Room for example). Bruce does not dispute this, however, Baker still receives no royalties. To a man, everyone interviewed in that film, including Bruce, Clapton, Winwood, Neil Peart, Lars Ulrich an on and on and on, Places Ginger in a class above everyone else, like Bonham or Moon. Clapton damn near choked on the question. He said something to the effect of " God no, Ginger was nothing like those players. Ginger had the gift, he could see music, arrange music and had perfect time. He was a fully developed musician, not just some drummer". Even his son, whom Ginger has treated like complete crap and hasn't talked to years says he is one of the greatest drummers to ever live.
His beefs with Bruce rarely had anything to do with his musical ability or skill as a player. He once pulled a knife on Jack, I think over Jack playing bass along with one of his drum solos. Ginger is a nutbag for sure. He got like 5 mil for that Cream reunion a few years back. Blew every penny on polo horses, which he was later forced to sell, along with his farm in S. Africa. As of the time this film was recorded he was dead broke.
Ginger also spent large amounts of money building an animal hospital in South Africa.