A German court has refused to extradite to the UK a man accused of drug trafficking because of concerns about prison conditions in Britain. The decision has been described as a “severe rebuke” and “an embarrassment for the UK” by a member of the Law Society. The case involves an Albanian man who lived in the UK. He was accused of trafficking approximately 5kg of cocaine and of laundering about £330,000.
It sounds like something similar applies even under the European Arrest Warrant, that each individual appeal can still make an argument that the justice system is flawed and so they should not be extradited.
https://www.politico.eu/article/european-court-belgium-reject-extradition-catalan-separatist-lluis-puig-spain/