I love New Advent as much as the next guy, but there are some small issues that one needs to keep an eye out for in using their 'Church Fathers' collection.
This is NOT to criticize the authors of the website as somehow lazy or ill-equipped for the task (far be it from me!); rather, many of the issues come down to problems of formatting, many of which are received from the Schaff set upon which the translations of the Church Fathers are based. Further, many of these issues occur only in certain works and not in others.
A few issues to keep an eye on,
In quoting many fathers, we don't simply quote "book, number" (e.g., lib. 3, n. 4), rather, we quote "book, chapter, number" (e.g., lib. 3, cap. 2, n. 4) or "book, chapter, section, number," etc. This is sometimes reflected in the Schaff (and, therefore, New Advent) and, at other times, it is not. This is especially evident in genres like letters, homilies, etc. To leave out these chapter/section numbers often makes it harder to do a divisio textus of the work and understand how the different parts of a larger work relate to each other.
In a similar way to how we have section/chapter headings in our bibles, many of the editions of the Fathers also have these section/chapter headings. Sometimes, these are included on New Advent, other times they are not.
There are MANY footnotes and parenthetical citations (especially from scripture) that are left out in the New Advent edition.
None of these three issues is grievous enough (or universal enough) to give a hard judgement against using New Advent, but certainly reduce the readability of the Fathers and, therefore, should be understood. If you are having a hard time reading a certain work from a Father, it may have more to do with the mode of presentation than it does with the content of the work.
One of the works that all three of these problems occur in is St. Augustine's On Continence. If you go to the Latin edition of his works (readily available on Augustinus.it) then you can easily gather the chapter titles and footnotes, along with proper numbering, so I incorporated these into a PDF since I find the work to be especially valuable (especially in this Holy Season of Lent!).
I attached the PDF below.
Ite ad Thomam,
Christian B. Wagner