For me, it's mostly the number of concepts and the fact that they need to be known upfront. None of them are particularly difficult, but I've done ops for a bit and gone from manual sysadmin on bare metal to Puppet to Chef to Salt... then when we started playing with docker, there's the initial learning curve, then a slew of orchestration options to consider. I'm exhausted keeping up with devops tools.
What I want is a better way to learn hands-on or as I go. Not sure exactly what that looks like.
Nothing reminds you how little you actually know when you have to pick up docker/k8s for the first time, and realize that it's a long journey to understand every nuance you need to get things done.