Glad that you like my article. I authored one book on API testing as well. IMO, the challenge of API testing is nowhere near automated UI testing. The reasons are simple: modern apps are dynamic. I have API tests for my own apps as well, frankly, the effort is too small that I rarely mention it (all my API tests are written in Ruby, a great and fun scripting language). The most effort for my team is Automated End-to-End testing (~70% of all development activities, including management, requirement, coding, testing, bug-fix, support, i.e., all), the benefits are GREAT. Life for a software engineer is much better when all end-to-end user stories are covered (not 100%, but satisfactory level).