For cost optimisation reasons, circleCI is costly solution and costs a lot of money, my Jenkins CI/CD runs on EKS cluster with AWS spot instances and cost a much lesser.

Jenkins is open source, meaning you have a full control over your workflows and all the things you do with CI/CD, so of course for me as Devops it's better solution - I want to control everything.

I think circleCI is better when you start and don't want to waste a lot of time on CI/CD, hire devops and so on. But it becomes pricey quickly, and soon you realize you want more control over your pipelines, to have some shared library for all pipelines...

Yep, but it's worth it - like I sad more control for advanced users:-)