FME Server has a very mature and complete REST API that is easy to work with:
https://docs.safe.com/fme/html/FME_REST/apidoc/v3/index.html
Security aspects are also well taken care of:
https://community.safe.com/s/article/fme-server-security
Based on personal experience, I would say that FME Server is perfect for back-end work, especially for "longer" jobs. I would probably not consider using it as a critical part of a responsive GUI, as there is always the possibility of long-running jobs blocking the engines, and thereby not giving the result within a reasonable timeframe. So for example using FME Server as a clip-snip-and-ship solution, I'd consider FME Server as an asynchronous service.
And if you want to consume api into workbench use the HTTPCaller