Tumours which secrete erythropoietin or substances having similar effect will stimulate bone marrow and will produce more red cells. These cells are normal in all aspects. So, in my opinion their function of oxygen delivery to tissues must be as good as the other normal red cells.

Tumors themselves can form RBCs in my view. Although it may not carry nucleus the RNAs in RBCs may reprogram phagocytes upon phagocytosis: so I would not consider it as safe. Here is some info on that

Article The Blebbishield Emergency Program Overrides Chromosomal Ins...

I agree with Ramesh Z. Patel