I'm curious about the experience of having taught a senior dev GenServers so early on. I guess it depends on what they are working on, but I think for most people (senior devs included) GenServers are YAGNI -- I have not once written a genserver for my current job, the senior might be tempted to be clever and use GenServers as code organization or as a circumvention to write stateful code, a la "objects" because GenServers are rather close to Alan Kay's original objects.
But I guess that's what code review is for!