Retrospective
JCasC is a powerful tool with active development still in progress. It gets a lot of the configurations codified, and while it's not perfect, it's almost there.
With the vast number of plugins in the Jenkins ecosystem, of course it's a challenge to wrangle every developer to make their plugins compatible with JCasC. It's disappointing when a plugin as popular as GHPRB still doesn't fully support JCasC, but GHPRB developers are aware of the missing feature and are working on addressing it3. For the people who really want to automate GHPRB configuration, the Configuration as Code Plugin - Groovy Scripting Extension plugin is available to fill the gap.
Also, Jenkins developers are working on the credentials import/export issue4, so I think that in a couple more years JCasC will reach its full potential. As a CI/CD engineer who uses Ansible and dozens of Groovy scripts to manage Jenkins, I eagerly look forward to JCasC being completed...