Many proprietary bank APIs remain unstable, Marten Nelson, co-founder and CMO of fintech Token declared last night.As the vendor of an "open banking platform [that] builds bank direct payment methods and data aggregation solutions for their customers," Token's key people and staff are in the thick of it.Nelson's barbed commentary draws on the firm's UK and European experience."Perhaps without appreciating the availability of external help, or out of a desire for control, the UK's main banks started out by creating their own APIs to allow third party access," he wrote. "In the process, they spent a fortune, duplicated effort, and ended up with something complex, costly and compromising. Things didn't exactly get off to a good start when six of the CMA9 - the nine biggest UK banks - missed the government's open banking deadline."In Australia, disclosure of API adventures is patchy. Westpac put its API development at the best part of 200 late last year.Token's Nelson said that "when they did begin to roll out APIs, inconsistent interpretations of standards resulted in a poor developer experience, and limited consumer use. The first wave of offerings were rudimentary bank-specific apps - primarily giving bank customers the ability to view multiple accounts in one place. "Worries about security breaches and data protection headaches also occupied banks so much in the early stages that many failed to see the broader potential."Banks and their partners still have some strategic decisions to iron out. Many proprietary bank APIs remain unstable, as I saw when we attempted a live demo of a TPP initiated payment at Merchant Payment Ecosystem in Berlin - which failed due to technical issues on the bank authorisation side."