The solus unus sanctus argument claims that a particular idea is either the teaching of the Church, or is at least a tenable theological position, solely because one Saint asserted the idea. This argument is not valid; the conclusion does not follow from the premise. The teachings of the Catholic Faith are found in Tradition, Scripture, Magisterium. The words of the Fathers, Doctors, and Saints of the Church are often a reliable indication of the teachings of Sacred Tradition, and a reliable interpretation of the teachings of Scripture and Magisterium. But their words are not Tradition or Scripture or Magisterium itself (except for those Fathers, Doctors, or Saints whose words were also an act of the Magisterium), and are not infallible.
As time passes, the Church grows in knowledge of the teachings found explicitly and implicitly in Sacred Tradition and Sacred Scripture. The Magisterium continues to answer questions on matters of faith, morals, and salvation, and to define new doctrines. Even the holiest of Saints cannot be expected to anticipate the future teaching of the Magisterium, hundreds of years in advance. So while a great theologian like Augustine or Aquinas might contribute substantially to the development of a particular doctrine, his words cannot be treated the same as a definitive magisterial teaching on the same subject.
It is an error to follow the opinion of only one Saint, while at the same time ignoring all other teachings in Tradition, Scripture, Magisterium. A person who does so sins by ignoring the teachings of the Church and by substituting the teachings of one Saint for all the teachings of Tradition, Scripture, Magisterium.