Out of the Famous Forty Oz books, I've only read Baum's fourteen and two of Thompson's. I have nothing against the other authors, I've either not gotten around to reading them, or the author has struck me the wrong way.
In the case of Ruth Plumly Thompson, I have been unable to really enjoy her Oz books. Let me shed some light on her background history...
Thompson was a writer for magazines and newspapers, churning out many short stories and poems for children. After L. Frank Baum's death in 1919, she was selected by Reilly & Lee, the publishers of the Oz books, to continue the Oz books. The choice was also approved by Maud Baum, L. Frank Baum's widow. She took the job, mainly to support herself and her invalid sister.
Thompson turned out 19 Oz books between 1921 and 1939, when she eventually ran out of ideas. However, she wrote a few new Oz books later, now published by The International Wizard of Oz Club.
Thompson's books have been both loved and scorned by Oz fans. There are are several reasons why.
Baum's original vision was not shared by Thompson. Baum envsioned a truly American fairyland, with few resemblances to the traditional European fairy tale. Thompson swerved more to the more traditional tale. Her Oz is full of miniature kingdoms with traditional princes, princess, witches, sorcerors, and boy heroes.
Thompson's writing style is more fitted to her fantasy work for the magazines and newspapers. Baum, on the other hand, could write in a variety of tones and styles.
Thompson was writing about something she'd read about. Baum created Oz.
With that being said, I think Thompson deserves less flack than her work has gotten recently. I do not consider her a hack, just not the best choice to continue the Oz books.
After all, you have to admit, others have done worse.