St. John is considered by many to be highly influential to later generations of illustrators dealing in Science Fiction and, especially, Fantasy art. That is probably more to do with establishing certain conventions than his abilities as an illustrator. In the post cited above I mentioned "reproductions of his paintings often strike me as having too-fussy brushwork." I think this tendency carries over to his monochrome illustrations, especially those rendered in pen-and-ink.
To demonstrate my point about St. John's dithering penwork, compare those illustrations to that of master pen-artists Franklin Booth (1874-1948) and Joseph Clement Coll (1881-1921). Some of the difference might be chalked up to the quality of paper used for printing their works. Book illustrations on the same kind of paper as the text couldn't support fine penwork. On the other hand, some books from the early decades of the 20th century had glossy paper tip-ins that allowed for much more detail and subtlety in the artwork. In such instances St. John would abandon pen-and-ink for charcoal or wash drawings.
Interesting fact: All the men mentioned above were near-contemporaries.
Here St. John does a better job on the lion than he does with Tarzan.
Some illustrations from another Tarzan book.
Here the depiction of Tarzan's body comes off fussier than that of the foliage.
This seems to have some wash or diluted ink work. Still, the pen strokes are largely haphazard, resulting in lack of clarity for the scene depicted.
Illustration for one of the John Carter of Mars novels. The background shading blends too closely to the definition of the subjects, reducing clarity.
This illustration was intended for slick paper, but St. John's use of shadows again hurts the composition.
All the images thus far, including this one, are probably scans from old books. This one seems to be from a slick tip-in, but it's hard to be certain what medium St, John used.
My previous St. John post included a reproduction of this image from a book, whereas what we see here looks like a scan of the original artwork. So far as I am concerned this wash drawing is his most successful illustration ever. The fussiness seen in the other images is absent. Why? I have no idea.