The position that one needs to watch the whole film to make a judgment only makes sense if you specify what you are planning to judge. If it’s the artistic quality of the film, then yes, absolutely. If it’s whether children were exploited to make it, you absolutely don’t.
It works exactly the same as in the case of the “mostly peaceful protests.” You don’t need to watch the 4-hour footage of the whole protest to decide if it was peaceful. A few burned buildings and assaulted passersby answer that question exhaustively.
“But it’s just one scene; the rest is good” = “but it’s just one burned block; the rest of the city is intact.”