The one extradition case we have seen play out, to a country famed for its liberal criminal justice policy (Sweden) from a country famed for the rule of law (UK) gave him many opportunities to test the process through the courts. That was even through the European Arrest Warrant process which is supposed to streamline extradition processes.

If, and it is an if, Assange were to be subject to an extradition request from the US he would be able to challenge it through the courts. He wouldn't face the death penalty as European countries don't extradite under those circumstances. Similarly prison conditions can be argued and again European courts can require commitments from the US regarding conditions and treatment before they would approve an extradition.

The Gary McKinnon case is illuminating into the thinking behind extradition from Europe to the US.

Finally it is also worth saying that Assange has probably also shot himself in the foot if he ever did face extradition to the US. He was bailed in the UK (where there is a presumption of bail) and this able to flee to the embassy. If he were arrested again facing extradition to the US he would be much less likely to be granted bail and so unable to claim political asylum on a much more sympathetic case.