tl;dr:Nicolas identifies 2 problems: (1) HTTP status codes were specified for human-to-machine interactions via browsers, not for machine-to-machine interactions via APIs. Hence, selecting a status code that maps one-to-one to the use case is rarely straightforward. (2) The error payload's structure is unimportant if a single organization manages the client and the API provider. However, the issue occurs when a team decides to use a third-party API.
tl;dr:Nicolas gives an explanation as to why for each: (1) Cache invalidation. (2) Naming things. (3) Dates, times and timezones. (4) Estimates. (5) Distributed systems.