Why would you trust a company no-man any more than a company yes-man? They both have agendas and biases. Is it just that you personally prefer one set of biases (anti-company) more than the other (pro-company)?
Yes, I am very much biased toward being anti-company and I make no apologies for that. I've been in the corporate world long enough to know first-hand the sins that PR and corporate management commits on the company's behalf and the harm it does. I find information coming from the individual more reliable than having it filtered through corpo PR, legal, ass-covering nonsense, the latter group often wanting to preserve the status quo than getting out actual info.
Because there is still an off-hand chance that an employee who has been let go isn't speaking out of spite and merely stating the facts - depends on a combination of their honesty and the feeling they harbor about being let go. Everyone who is let go isn't bitter and/or a liar.
However, every company yes-man is paid to be a yes-man and will speak in favor of the company without exception - that literally is the job. Otherwise they will be fired and will join the ranks of the aforementioned people.
So logically it makes more sense for me to believe the former more than the latter. The two-sides are not equivalent (as you may have alluded) in term of trustworthiness.