I don't know about IEEE access but I can vouch for MDPI journals (at least the ones I've published in). MDPI's review is robust and they provide feedback in a timely manner. I have had some of my work soundly rejected by them and they have always been very kind in their correspondence and critique.
From my experience (an author and a reviewer), MDPI journals have a conventional review process, no any differences. "Reject" and "major revision" are pretty common.