Participation is limited to tertiary academic institutions, and possibly only four-year (rather than two-year) ones. This excludes local (city/county) libraries, as well as primary/secondary (grammar / middle / high school in the US) libraries.
Even public-domain records cannot be downloaded in whole, but rather can be saved one page at a time as PDFs. I'm pretty sure that those interested in more useful archival will and/or have created automated tools to do so, but HathiTrust remains the most notable point-of-access for such works, and the additional generation of conversion and republication further degrades the quality of original-publication formats. (It's less a problem for regenerated works from OCR'd or manually-converted documents, but those of course lose all the characteristics of original publication.)
And of course, many materials still under copyright are not accessible to the general public at all, no matter how obscure. I'd run into a case of this some months back trying to get a date attribution of an Alan Watts lecture which had been posted to HN:
<https://news.ycombinator.com/item?id=41231047> (thread).
And my request still stands. Anyone with an academic affiliation who can check <https://catalog.hathitrust.org/Record/000678503> and see how it relates to this post (<https://news.ycombinator.com/item?id=41230841>) would have my gratitude.