Adyen don't look very professional.
I'm sure if they smartened up their website and offered a REST API they'd be in a much better position for web based startups to consider them, otherwise I'm left searching for signs that its a professional and trustworthy company to try and balance my initial impression.
But what I'm trying to figure out is if design and a REST API are the only 2 things where GoCardless outexecutes competitors. Especially since direct debit clearly isn't something new and there are other players in the field that offer a much broader support for different kinds of payment methods.
I hadn't come across Adyen until you mentioned them here.