Has he been exposed making unfounded accusations in the past?
Hard to say, there have been times where various rumors of coming leaks have cropped up and never come to fruition, but it is hard to lay that solely at Assange's feet.
What we do know, however, is that many people who have tried to work with Assange have come away with negative opinions of him as a person.
His relationship with the NYT and Guardian broke apart because he wouldn't work with them on redacting info that could endanger lives.
A documentary film maker who made films about the Catholic Church and Enron and was sympathetic to Assange balked at paying him $1 million to be interviewed. The director said 'he's become a spin doctor, more comfortable telling lies than the truth.' An editor on the film who tried to negotiate his involvement had paid 24,000 pounds of his bail which was lost when he fled. Her opinion on him soured after trying to work with him when he demanded editorial control over the movie.
The ghostwriter of his autobiography had this to say, after Assange backed out of the book deal: "he is thin skinned, conspiratorial, untruthful, narcissistic, and he thinks he owns the material he conduits. It may turn out that Julian is not Daniel Ellsberg or John Wilkes, but Charles Foster Kane, abusive and monstrous in his pursuit of truth that interests him, and a man who, it turns out, was motivated all the while not by high principles but by deep sentimental wound. "
A 2010 New Yorker profile quotes Assange as saying that 'his mission is to expose injustice, not to provide an even-handed record of events.' To him, 'leaks were an instrument of information warfare.'
I personally have great respect for Edward Snowden, but I think Assange is just a narcissistic piece of ####.