The concerns often revolve around:
* High publication fees: MDPI journals frequently charge relatively high Article Processing Charges (APCs). While this isn't inherently predatory, it can be a red flag, especially when combined with other questionable practices.
* Rapid publication times: While speed can be a benefit, exceptionally fast publication times can indicate a lack of rigorous peer review.
* Aggressive solicitation of manuscripts: MDPI has been criticized for its aggressive marketing and solicitation of submissions, sometimes targeting researchers directly with personalized emails.
* Varying quality of peer review: The quality of peer review across MDPI's journals varies considerably. Some journals maintain higher standards than others.
* Lack of transparency: Some concerns have been raised about the transparency of MDPI's peer-review process and editorial practices.
Therefore, while MDPI itself is a large publisher, a blanket statement about its predatory nature is inaccurate. Researchers need to carefully evaluate individual MDPI journals before submitting their work. Checking the journal's impact factor (if available and relevant to the field), assessing the editorial board's reputation, and scrutinizing the journal's publishing ethics are crucial steps. Consulting Beall's list (archived versions are available online, though the original list is no longer maintained) and other resources that track predatory journals can also be helpful, keeping in mind that such lists are not perfect and should be used with caution. In short, some MDPI journals are legitimate, but caution and critical assessment are always advised.