Author here. We are in beta and have about 40,000 cards issued to customers in the UK, with a waiting list of about 200,000. I think we have good reasons for making the architectural decisions we have made, some of which are laid out in the post, but perhaps I did not explain them in enough detail.

For example, we chose HTTP as our RPC protocol because it is widely supported in many languages, and has mature support in Finagle. HTTP/2 fixes most of the performance issues with HTTP, and we have options (eg. to use mux) if we still find there are performance problems – although at the moment, we are quite happy with performance.