Are there health reasons for not using hot liquids in Ninja blenders, such as leeching of chemicals from the plastics?
I suggest you avoid putting hot liquids in any of the Ninja blenders that do not allow air to leave the blending vessel. Models like Nutra Ninja Pro, for example. When you blend a hot liquid in a closed system, it creates a lot of pressure and you can easily make a mess or injure yourself with flying hot liquids.
We are not health experts, so I cannot comment on leeching chemicals beyond pointing you to fact that Ninja claims their products are BPA free. For details beyond that, I suggest contacting the manufacturer and asking for their recommendations in regards to hot liquids.