I certainly agree that testing these integrations is unnecessarily painful, but given that both Stripe and GoCardless offer much the same facilities -- a single sandbox environment, but without comprehensive simulation of all important scenarios and with nothing useful for either automated integration testing or offline simulation at the developer's desk -- I'm not sure I'd say either is much better than the other. The only major difference I can think of is that since they simulate roughly realistic timing as well, GoCardless can also take several days to complete a test run for a new integration or bug fix, which is obviously absurd.
