I find it difficult because of understanding how to get it to fit with an existing directory layout. And maybe existing scripts in the build. But yes, setting up the tooling is overwhelming.
create-react-app says in the README:
"If you need to integrate React code with a server-side template framework like Rails or Django, or if you’re not building a single-page app, consider using nwb, or Neutrino which are more flexible."
So I go and look at nwb and Neutrino, and there's nothing there going "Need to use us in a legacy project with its own directory structure? Here's how".
I feel I'm missing a piece of the jigsaw...
