What type of junglers would gain the most benefit of getting Feral Flare over other items, and what are the trade offs they would face?
Mostly auto attack junglers or very quick clearing junglers seem to receive the most benefit from this. People like Master Yi, Warwick, Tryndamere, Aatrox, Nocturne, Shyvanna etc.
The main disadvantage is that to make the item worthwhile the junglers will typically heads down farm the jungle in order to build the stacks to turn it into a FF. This can lead to the other jungler getting many early ganks and winning the game before you can get strong.
Typically, the better junglers for this clear extremely quickly. That is why Master Yi has been recently a large pick. An alternative to that is an extremely mobile jungler that lets them counter gank easily. Nocturne is a great candidate for this as he can heads down farm, but if he is needed his ult let's him quickly get to the fight. You can then build him full tank and he still does great damage with the FF + 1 other attack speed item.