FF is stupid. Its OP because it is not only too easy to stack, it, along with BORK gives insane mixed damage sustain and Wriggles is stat efficient early while giving good ganking stats for a very low price. All the junglers who can best use it are the ones who don't spend much time ganking or have massive enough move steroids they can rapidly gank before slipping back into the forest to continue farming.
Its similar to what Meteos stated, he isn't a 'farm jungle', he doesn't play to carry. He just ganks very, very efficiently. The problem with most people who play 'gank' jungles is they second guess. They posture, they think about ganking. Ganking well is about predicting what lane will need you when, and being in the area while farming to gank/countergank.
The fact is? I had a feral flare by 18 minutes as Evelyn while ganking whenever I had an opportunity to do so. I wasn't farming all that fast, but I would always take camps whenever I could. If I knew an enemy jungler was away, I'd take his camps. I'd take my camps while waiting for that moment to gank. I was 4/0/2 by eighteen minutes. Want to know the only reason why I wouldn't go Feral Flare on Eve? Elder Lizard's generally superior due to her kit and Feral Flare gives her some health and mana sustain issues Elder Lizard doesn't give her. It did give me some crazy dueling power but it just didn't match Eve's kit.
Eve isn't a farm jungler. I also wasn't 'useless' before 20 minutes. I had constant impact and applied pressure often.