I cannot be sure if Nechung's unreliability is related to either the Oracle is being entered by a Malevolent spirit or Nechung is making unreliable predictions on purpose.
What is absolutely clear though is that the current prophecies of Nechung bring harm to those who follow them.
Then on the other spectrum, you have Dorje Shudgen, yes kind of the new kid of the block in terms of being a Dharma Protector but an emanation of Manjushri, with a string of lamas predecessors whose reputation and benefit to sentient beings ares stainless. Dorje Shugden who received praises from the Emperor of China, multitudes of Lamas from all 4 traditions, 5th Dalai Lama and 14th Dalai Lama, yet there are those who criticize Dorje Shugden.