The concept of that “the simpler the form of a letter the simpler its reading” was an obsession of beginning constructivism. It became something like a dogma, and is still followed by “modernistic” typographers.
This notion has proved to be wrong, because in reading we do not letters but words, word as a whole, as a “word picture.” Opthalmology has disclosed that the more and the letters are differentiated from each other, the easier is the reading.
Without going into comparisons and the details, it should be realized that words consisting of only capital letters present the most difficult reading – because of their equal height, equal volume, and, with most, their equal width. When comparing serif letters with sans-serif, the latter provide an uneasy reading. The fashionable preference for sans-serif in text shows neither historical nor practical competence.
Josef Albers, Interaction of Color