A devourer into standard ADC build was pretty mediocre. The early clears were weak at best because of low dps,, and devourer was a pretty lame power spike because he synergizes so terribly with on-hit damage. Ganks were pretty pitiful. Hit the highly telegraphed W skillshot or do literally nothing, and possibly do nothing even if you do hit it. Doesn't have the reliable burst of graves or the sheer tanking power of quinn to fall back on, and I really think that if the Amumu I was against was more aggressive, there wouldn't have been a single thing I could have done to stop him counterjungling or killing me. The dueling power just does not exist.
He can clear the jungle, and that's all I can say for him. It's not impossible to pull it off, but I'm pretty sure other junglers will always be strictly better than him.