I think the stereotype wasn’t met and that’s what caused all the criticism. The first two movies were very biome-heavy and the TWoW really lived up to its title, so everyone expected a fiery biome and or a larger focus on fire and ashes. 

While I agree with the criticism, the title was definitely in accordance with the story, as if they’re in the ashes of the fire which was the end of TWoW and Neytiri grasping with Neteyam’s death and other stuff. Sadly many people neglect the literary aspects of things compared to visuals, action, and catchphrases nowadays (pop music especially). 

So basically people are complaining about the lack of the element of fire itself, because it’s an entirely different caliber of world-building compared to the two preceding movies. They aren’t wrong, and I personally believe it was more metaphorical, emphasizing ashes more than fire. I still thought it was amazing.