People often harp on about TAS somehow being inauthentic, but compare it to most other live-action-to-animation transfers of the 1970s and 1980s. Almsot none of them made the transfer to animation without making substantial changes to the mythology of the parent live-action series, usually even "dumbing down for kids" (as if kids need something to be dumbed down when they enjoyed the live-action version perfectly fine as it was!).
Filmation was often pretty good with doing faithful adaptations. Sure, they did a few that changed things drastically -- The Brady Kids
dropped the parents from The Brady Bunch
and added talking animals, magic, and time travel, and My Favorite Martian
became My Favorite Martians
and was centered around the teenage relatives of the original show's stars. But others were much more faithful. The New Adventures of Gilligan
kept the original premise and cast pretty much intact, although the focus shifted to be more moralistic; rather than Gilligan being the bumbler who ruined all the rescue plans, he became the innocent whose purity saved the day when the others got led astray by assorted vices and temptations. (Although then there was the sequel series Gilligan's Planet
....) Filmation's Tarzan, Lord of the Jungle
was just about the most faithful screen adaptation ever made of the original books, just toning down the violence (and maybe updating the timeframe, though that was unclear). Their Lone Ranger
were pretty faithful to the sources too, though the former played fast and loose with history in order to teach about history, since the Ranger and Tonto got involved in real historical events that spanned three decades yet they didn't age a day.
Sure, some of the criticisms are valid. There is limited animation. The half-hour format does truncate storytelling compared to the hour-long format. The oft-quoted color mistakes are rampant in some places. But these are only nitpicks when TAS, at it's very core, is as much Star Trek as any other series.
Heck, it's not like TOS didn't have similar problems. The effects were comparatively crude and often had mistakes, the sets and alien makeup weren't all that elaborate sometimes, there was a lot of stock footage, there were too many gratuitous fistfights, etc.
The voice acting is mailed in...
I'll concede that point, especially since it's pretty much literally true; the actors were mailed the scripts, recorded their lines at whatever studio was convenient, and sent the recordings in to Filmation. Due to the lack of direction, the actors who weren't experienced at acting with their voices alone didn't give very good performances. The ones with voice experience, like Doohan, Takei, and Nichols, did better jobs.