As an academic that has published in, and reviewed for several MDPI journals, this total nonsense- they are a legitimate publisher. I haven't seen any signs of them being corrupt, predatory, or dishonest. Many of their niche journals are well respected in narrow sub-fields, and seem to be good quality. They also have very modern publication/typesetting/submission workflows that make them a pleasure to work with vs the older closed access journals. I would say they have lower standards for 'impact' vs more prestigious journals.... which means they play a valuable role in the field- they publish mostly technical methods, and other results that are interesting and useful to a narrow specialized audience.