We have received pleasant feedback from a long-time customer, who just begun a custom backend integration of BankID into their Android and IOS mobile application. They used the server-2-server API, and the process could be worked trough in a day or two, without major hassles. Hopefully thanks to this online documentation (here) and C# code sample (here) as well as quick response time from us.
One of our main focus areas is Ease of integration – we are doing things right!
CTO at ZignSec