As a documentary i found it to be rather mediocre and a bit on the amateur side (the production value, editing, research, and overall presentation of the material), but i really enjoyed watching it regardless, including hours of the bonus material, even if i had already seen so many of the clips online over the years. There wasn't really anything revelatory, but its strength lies in rounding up the principals and getting them to share their anecdotes and memories. Sadly, these musicians have steadily been leaving this earth and will only continue to do so in the next decade or so, so i think it was an important film to make, sooner rather than later. That said, it felt more like seeing some old friends again rather than watching a good documentary, which i suppose is not necessarily a bad thing. I loved seeing Tommy, Hal, Carol, and Plas sitting around a table reminiscing, telling stories and jokes, laughing. Same with the Goldstar staff. Glen Campbell is a joy, really charismatic. Cher has a great spirit and i was surprised and very touched by what a strong connection she had/has to all the musicians considering she was a sort of inside outsider in that scene. Leon Russell, i could watch hours of him speaking, what a guy. Earl Palmer is not having any of that sh*t, as was expected from him.
I do think Denny Tedesco was remiss in not dedicating more time to Phil Spector and any time at all to Jan & Dean, as those two were pivotal in bringing the WC into prominent use with more popular/rock & roll artists in the early days. But what really bothered me about the film is that most casual viewers are no doubt going to walk away believing that the Beach Boys didn't play on most (or even any?) of their records. I'm afraid that myth is not only not going to die anytime soon, but it's probably going to gain even more traction with this movie reaching mainstream audiences. The director really sold the group short as musicians, and although i was expecting as much, it was still frustrating to actually see the myth perpetuated in the finished film. I also thought some of his editing choices were bizarre. For example, while "Good Vibrations" is showcased, he pans across a photo of the Surfer Girl LP cover? Uh, ok. Is that supposed to demonstrate the incongruency between the public image of the band and the "reality" of the studio situation? It's a baffling choice no matter how much you try to justify or contextualize it.
The most bothersome aspect of the whole BB/WC thing, of course, is that the story could easily be told factually without taking any credit away from either the band or the session musicians. Those musicians played on the near entirety of Pet Sounds, a record universally hailed as one of the best and most influential albums of all time, and Smile, possibly the most mythical unfinished album. It's not like there isn't a journalistic angle to work this story. It's there. And in that case, i don't see what the controversy is. I don't understand why or how anyone would hear horns, woodwinds, strings, mallets, orchestral percussion, bass harmonica, accordion, and theremin and think that the band members could and would play all those instruments. Was it really revelatory and disappointing to Dick Clark and the general public back then to learn that five young guys in a garage band from Hawthorne, California weren't playing the "auxiliary" instruments on their records? And going back to 1964 when the crew started appearing on their records with more regularity, was it surprising that a band who was releasing 3 albums per year while touring the globe would occasionally use session musicians to augment their tracks, especially given that their leader was staying off the road to, you know, produce records?
Couldn't cover this better, so why try? Like you mention, the most baffling thing was Good Vibrations playing over the Surfer Girl cover. I understand the visual appeal, but that was truly a weird choice. It was entertaining, but it did come off very amateur at times. Also, Mickey Dolenz equating the Monkees lack of playing to the Beach Boys and being unchallenged was disappointing.