Ravana admonishes Vibhishana

Vibhishana informs that none on the demon's side, including Ravana, or Kumbhakarna, or Indrajit, etc, can withstand Sri Rama, in battle. 

Then Indrajit criticises Vibhishana to be the only individual born in their race lacking in strength, heroism, prowess, courage, valor and vital powers. He questioned the veracity of Vibhishana’s statement that he, who suppressed the arrogance of Devatas, was not capable of conquering Sri Rama and Lakshmana, who are ordinary human beings. 

Hearing those words of Indrajit, Vibhishana spoke following words: 

"O, child! Being a boy, you are still immature in mind. That is why many a meaningless word was prattled by you too, for only your self-destruction.”

 “Who can endure the arrows, discharged by Rama in battle? Restoring back Seetha to Rama, let us live here, free from anguish."

Ravana, goaded by death, admonished Vibhishana, as follows:

"One can stay together with an enemy or even with a furious snake, but one cannot stay together with an adversary yet avowing himself as a friend.”

“For us, fire or weapons or nooses are not begetting fear. The terrific kinsmen, incited by selfishness are begetting fear to us. These kinsmen will inform the strategy to our enemies to seize us. There is no doubt about this. Out of all fears, the fear of kinsmen is very much troublesome for us.”

“If any other person utters these words in this manner, he will be ceasing to exit at this moment."

The very purpose of calling for the Assembly is to know the views of demons and bringing them together in view of ensuing war with Sri Rama and his army.  But Ravana, due to his ego, could not stand to criticism.

Except Khara, Dhushana and their followers nobody from Ravana side faced Sri Rama in the battle and they were no more alive.  Then how had Vibhishana described Sri Rama’s strength?

Scholars say that (i) a wise man will learn from the lessons of others, (ii) a mediocre person will learn from the his own experiences and (iii) a fool will never learn.

I think, being a wise and the follower of Dharma, Vibhishana understood the strength of Sri Rama from the narration of Shurpanakha, his sister.  Further, Sri Hanuma also described the strength of Sri Rama in the Assembly of Demons.