It's an immediate 50% AS boost with 40 magic damage base per attack, which goes up on kills, assists, and large monsters, giving 2 bonus damage on the first two. Have you seen Yi with it yet? Or even Shyvana. The healing started at 10 on Feral Flare, and so wasn't the issue early - the issue early was a ~2000 gold item that gave 35% AS with bonus damage to spare. It's an immediate boost that the next competing item in cheapness is Wit's End, and a fourth of the benefits that item gives won't give the user any dueling capabilities against their AD counterparts. Devourer is 400 less and has scaling magic damage, which within 20 minutes I've brought to over 100 damage per hit. The main issue isn't the item - it's the cheapness of it, making it an easy pick-up to roflstomp teams early. I've caused more snowballing teams than I can count, and this is with Nautilus.
You know, that champ that not even 4 days ago was considered to be outside the meta? I've straight solo'd Udyr with Naut. Several times.
It's freaking Udyr. Against Naut.