There are a only few.
Signing up is not as easy as it should be for the customer. Not a problem for me but, for example, my dad couldn't do it. Many of my customers are either over 55 and not interested in giving out their details online or paranoid 'data security' types who don't like giving details to companies they have never heard of.
Referencing customers is a bit of an issue. i.e. I have only sent out requests for customers to sign up to GoCardless to my overdue and known bad payers (a relatively small list) and still I have had mix ups because you can't force references through (just email addresses, I don't have email addresses for all my customers and email addresses are subject to change). If I were to send out requests to sign up to GoCardless to ALL my customers the resulting chaos would have me bankrupt in a fortnight. I am currently paying someone to integrate the GoCardless system into my customer management software. This is likely to be costly.