The company I currently work for is in the process of migrating out of Jenkins to Github Actions. With all problems that Github has, it has been, by far, a much better experience, even with all these issues and trade-offs, Github Actions in combination with the Github UI has been a net positive in all aspects.
Jenkins is slow and a nightmare to maintain. It became a huge ball of mud that nobody wants to touch. Just keeping the lights on it's a large burden for the infrastructure team.
A bit a false dichotomy: there are other open source CI alternatives that are way more modern than Jenkins: Drone/Woodpecker, Gitlab, sourcehut all come to mind.
Jenkins is slow and a nightmare to maintain. It became a huge ball of mud that nobody wants to touch. Just keeping the lights on it's a large burden for the infrastructure team.