It seems the biggest critic of John R. Neill's work on the Oz series was L. Frank Baum himself. While he probably trusted his publishers with their decision to have the staff illustrator illustrate the Oz books, Baum wasn't too happy with the artwork. To be sure, he recognized Neill's talent, but he felt the Oz books should be illustrated humorously.
Baum wasn't very vocal about his dislike of Neill's illustrations until 1915, when he discovered The Oz Toy Book in a Reilly & Britton catalog. The book was thick pieces of stiff bristol board with the Oz characters from The Marvelous Land of Oz to The Scarecrow of Oz printed in full color on them, ready to be cut out. With bases to stand on, the characters could stand on their own when folded.
Baum sent a letter to Reilly & Britton, demanding why they had allowed Neill to infringe on his rights.
Neill had not of course, the book was entirely the idea of the publishers for an extra item to sell to publicize the series, as well as to prepare the readers for the new characters in The Scarecrow of Oz (the Ork is seen on the last page), which many of them would be receiving for Christmas.
Baum's experience with W.W. Denslow made him refuse to share the copyright of his characters with an artist, as he had done with Denslow, who practically owned half of The Wonderful Wizard of Oz.
Reilly & Britton apologized, and assured Baum that Neill was not to blame, and was not receiving royalties from the book. As a result, The Oz Toy Book saw only one printing, and today, it is hard to find a copy of the original edition of the book completely intact. Books of Wonder is currently selling a copy for $8,500, but for anyone who just wants to see Neill's work, the International Wizard of Oz Club offers a $6 black-and-white reprint. (I once started scanning this in and coloring the pictures on my computer, but never finished.)
The publishers also told Baum they "would impress upon him (Neill) the necessity of having the pictures as humorous as possible."
The next two books, Rinkitink in Oz and The Lost Princess of Oz, didn't offer too many humorous situations for Neill to illustrate, but in The Tin Woodman of Oz, Neill started to get a little more "humorous." The Scarecrow does an inverted bow when they meet Mrs. Yoop:
The Scarecrow's Song falls on Tin Ears:
Jinjur's cleansing of the transformed Woot definitely contains humor:
And the story lent itself well to humor, just look at this:
Baum, however, had resigned himself to the fact that Neill was his illustrator, and even wrote to Neill to offer him to visit so they could work on the Oz stories and illustrations together. "I'm sorry not to have met you personally for so many recent years...as I remember our former foregatherings with real pleasure and think we could harmonize if we were jailed together in the same cell," he wrote, though the mystery is, what "former foregatherings" was Baum referring to? No one has turned up any record that they had ever met.
But, however, the demand for more humor took it's toll on Neill's art. The detailed style so many previous Oz books had enjoyed was now lost. Compare this art from The Emerald City of Oz...
To this art from Glinda of Oz.
(References, introduction to The Oz Toy Book, International Wizard of Oz Club edition, by Barbara S. Koelle, and L. Frank Baum: Creator of Oz by Katherine Rogers. All artwork hosted from The Ozmatron.)