Luna is way different because of glaive bounce, Medusa too because of split shot. You're comparing crab apples and red deliciouses.
I'll admit Shadow Fiend can farm fast, but heroes like Medusa and Luna simply get more out of the same items than Shadow Fiend does. A Manta Style on Shadow Fiend means a stronger team fight presence - a Manta Style on Luna means split pushing, farming both jungles, or farming a huge stack of ancients.
Shadow Fiend performs best when played as a 2, or mid. In The Defense tournament, Kaipi versus Alliance, Kaipi fielded Shadow Fiend as their mid twice - once against Magnus and once against Timbersaw. Because of the way his damage scales he can dominate in LH/denies, and if he gets a BKB he's looking at dishing serious damage between minutes 20 and 30 with minimal farm. I'll admit I think that's pretty good, even if it's technical. It just seems, to me, harder to pull off than a Puck or QOP mid.
Basically my point is he's not a hard carry. He's a midgame carry at best who gets massively overshadowed by lategame carries unless he secures a powerful advantage early on. Which can be the case if he's being played by someone like KP.Arteezy.
Warlock might not be OP per se, but he's definitely the best team-fight hero right now. And given that team-fights are one of the most important parts of the game, that counts for something.