Gocardless has an API to make direct debit fairly easy for most of europe.
It's only really good for subscriptions rather than one off payments though. Not knowing if the payment was successful for 5 days is probably the biggest downside. Very low fees and low churn is the upside.
If you're talking about B2B it's not just subscriptions - any continuous relationship can be a good fit for direct debits. For example many breweries in Ireland will sign up their clients (i.e. pubs and off-licenses) via direct debit. Then if they make a delivery the payment collection will be automated at the end of the month.
In this situation direct debit gives the vendor the comfort of delivering goods without payment, and it gives the customer the flexibility of ordering without having to go through a purchasing process.
It's only really good for subscriptions rather than one off payments though. Not knowing if the payment was successful for 5 days is probably the biggest downside. Very low fees and low churn is the upside.