People are subject to illusions, hallucinations, mass hysteria, drugs, dreams, alternate states of mind brought about by stress or extreme emotions. That's why subjective observations are not considered reliable evidence. That's not to say such things couldn't be true, or useful, it's just that without independent verification, there is just no way to be sure.
The data itself is usually not biased, but that is also possible. Scientists benefit from disproving each other. The idea is to produce a solid case for your thesis, and collect the data using rigorous methods that negate any possible personal feelings on the part of the researchers. I would say that yes, biologists on both sides of the evolution debate do critique each other's work.