Um, I'm writing about the FILM. The FILM is different from the NOVEL. In the NOVEL, Fredo was rather successful, but this forum is about the TRILOGY.
In the FILM, Fredo goes to Vegas, has sex with so many waitresses that the gamblers can't get a drink, and gets slapped around by Moe Greene, making the Corleone Family look weak. If he's so busy with the cocktail waitresses, when is he learning the business?? Keeping your gamblers thirsty and getting beaten in public doesn't sound like a rip-roaring success to me. If he was sent to Vegas to learn the casino business, and he didn't, then, yes, I would consider him a failure.
In the NOVEL, Fredo takes very well to the casino business and IS quite successful. You are quite right on that point.
Oh, now I remember. yeah, the scene where Fredo gets off the plane at the Vegas airport, goes right to the casino, and starts having sex with the waitresses in the casino. Wow, with all that happening and so quickly, of course Fredo could not possibly had time to learn the casino business. And Moe slapping him around: "Hey Fredo, how come you asked Moe to slap you around?"
