Xenografted human tumors in mice != human cancer. The support structure of the tumor (tumor microenvironment) differs between model mice and humans, cells derived from human cancer that can be cultivated in a lab and xenografted differ from typical human cancer cells, and xenografting requires immunodeficient mice, just to name a few factors that affect treatment response.

Mice models of cancer are useful, but you should never be too surprised when something that works in mice doesn't work in the clinic, xenografting or no. Cancer is complicated.

Doesn't harm the mouse. But would it harm the normal human cells?