This is unrelated to the China thing in my opinion. No one sensible is claiming things like "the US justice system will be unfairly stacked against her" or "she won't be properly guaranteed her human right against <X>". In general though I disagree and wish our extradition system worked differently.
It doesn't seem fair to the accused to send them to a country with different (in some cases lesser) protection than our own for various rights for their trial. In fact, doing so seems like a direct violation of human rights and thus (in Canada's case) the charter of rights and freedoms.
It's also not necessary. If we want to save on legal costs, we should simply run a single trial in the extraditing country which complies with both countries standards of human rights, proof, evidence, law, and so on.