These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Pumpkin's revenge against Sayuri is meant to be a major Kick the Dog, but as she points out to Sayuri, Sayuri took Pumpkin's life goal of becoming officially adopted by the okiya and becoming one of the most successful geisha in Gion, and she didn't really even want that; it was all just "a stepping stone" to get to the Chairman. Under those circumstances, it's easy to feel like Pumpkin's revenge was justified, even if Sayuri hadn't meant to ruin Pumpkin's life.
Similarly to the Pumpkin situation, Nobu's reaction to Sayuri could be seen as extreme, but she did make him believe she loved him, under instruction from Mameha, so his feelings of betrayal when she and the Chairman get together is pretty justified, especially as the Chairman pretty much orchestrated the whole thing.
The Woobie: Pumpkin. Not as pretty as Sayuri, not as talented as Sayuri, not as smart as Sayuri, not as lucky as Sayuri, and browbeaten by her supposed mentor Hatsumomo into the bargain. Oh, and Sayuri is unwittingly responsible for Pumpkin's downfall.
Chiyo, at least until Mameha decides to take her on.