UK based here, so for most projects I/clients want cards as the primary payment method due to the audience as they are one-off payments.
I've used GoCardless on one project; as the developer I didn't enjoy the experience at all. Setting up testing for all states was painful (as was getting items to the right state to use their scenario simulator). I see Stripe have rolled out UK Direct Debit support now; hopefully their documentation and testing story are better than GoCardless.
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.
I've used GoCardless on one project; as the developer I didn't enjoy the experience at all. Setting up testing for all states was painful (as was getting items to the right state to use their scenario simulator). I see Stripe have rolled out UK Direct Debit support now; hopefully their documentation and testing story are better than GoCardless.