Published on the preprint server bioRxiv, Clarissa F D Carneiro and colleagues present the results of their study of the quality of articles published in PubMed-indexed journals versus preprints on bioRxiv. Using an online questionnaire, created on the basis of existing reporting guidelines, checklists, and earlier studies, Carneiro and colleagues found that preprints are generally similar to peer-reviewed articles in terms of quality of reporting; however, peer-reviewed articles are, on average, of higher quality. Results of their exploratory analysis showed that preprints with figures and legends embedded within text had reporting scores similar to PubMed articles.
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https://slate.com/technology/2019/06/science-replication-conservatives-liberals-reacting-to-threats.html
The article quotes researchers claiming: "We Tried to Publish a Replication of a Science Paper in Science. The Journal Refused."
As long as this kind of biases exist in the peer-reviewed literature, preprint servers seem to me mightily useful.