Disclaimer: I respect Resin and the service they provide developers, which I is why I wanted to join in the first place. This is only a review of their interview process and not the company as a whole.
I was invited personally to an online coding test straight away. The instructions encouraged me to fully answer the questions with proper documentation and testing, to show my process. The questions were easy (mapping lists, vanilla JS classes, etc) but the time limit was too short in my opinion to complete everything with proper docs and tests, including edge cases.
Since I didn't have a chance to actually speak with anyone, I tried to follow up with my interviewer about challenges at the company and trends in microservice and client architectures, etc, but what I received in reply was a form rejection letter and no further interaction.
My advice is not to approach these timed tests as an engineer solving real-world problems (designing in advance, documenting and testing as you go...), rather, as a competitive programmer (memorize test problems, hack out code, test after the fact, <insert more bad habits>) . Although I'm not sure what that says about the teams you end up working with.