[MikeWaters]

While Randomized Controlled Trials are subject to less bias than observational studies, they are not immune, by a long shot.

While the main analysis may not use demographic variables as covariates, certainly there are a variety of statistical methods that can be employed, each with a different result. They cherry-pick the one that supports their claims. Not to mention changing the rules at the end, including and excluding certain participants. Normalizing the data, not normalizing it, etc.

Medical scientists are generally woeful at quantitative methods, because they are generally not trained in it. They usually rely on biostatisticians, and they exert their influence on these people. "Is there a different way we can run it?"

Data collection can also be very problematic. When some underling doesn't follow procedures and makes up data, or leaves a huge gap that someone has to decide what to do. "Patch it up."

And no, there is really not any incentive to trash someone else's work, other than promoting your own, if it happens to differ. I can't apply for a grant to double check an investigator's work. The peer review process is problematic as well. The manuscripts get sent to people that often lack methodology expertise.