Assange has been subject to extensive illegal surveillance by the US government, while inside the Ecuadorian embassy, which also included access to private communications with his lawyers. This on its own would already be a reason to deny extradition, since it fundamentally undermines his capability to organize a proper defense and thus to get a fair trial.
There has been no proof that the surveillance was conducted by the US government, and some of the evidence provided by Assange's team indicates that both the UK and Ecuadorian governments conducted surveillance on Assange as well.
Moreover, Assange's team would need to show that the surveillance would actually prejudice his ability to get a fair trial, which is a tall order because the UK laws on this are less favorable for defendants than they are in the US. And if it turns out the surveillance was illegal and would prevent him from getting a fair trial, he can raise that before the U.S. trial court...and get the charges dismissed with prejudice.
