I saw a major show of John Singer Sargent's many years ago at the Boston Musuem of Art. It included portraits and some of his work from World War One. As is always the case, the original artworks were stunning beyond description - no matter how good the quality of reproductions.
In some cases, even in larger than life canvases - a single brushstroke "made the painting" come alive with light or shadow.
Personally, I've always been attracted to John Singer Sargent's watercolors from his travels thoughout the world. In particular, his ability to capture a sense of light and atmosphere. I have no idea of whether or not he ever intended to show his watercolors or if they were simply done for enjoyment or study purposes - I'm just glad that they have been preserved.
Even a mundane (or potentially mundane) subject comes to life under his brush. A simple white building sparkles with light as seen below.
Bedouins
Like another artist I admire, Aiden Lassell Ripley, John Singer Sargent was not "stuck" on tradition. Even though much of thier commissioned work would likely be considered to be traditional, when traveling or painting for their own enjoyment, both artists painted beautiful yet substantial impressionistic watercolors. Although the bulk of his watercolors appear to be translucent in nature, both Sargent and Ripley did utilize opaque watercolor (gouache) to achieve the highlights they desired. It was the effect that was important - not the rules for getting "there."