I used Jenkins for years at a previous job - for the longest time it was a confusing mess of pipelines coupled with being a fairly outdated version.

Once it was updated to latest and all the bad old manually created jobs were removed it was decent.

This. In my experience the people actively disliking it have only ever used Jenkins 1 or somewhy only used freestyle jobs.

There are numerous ways to shoot yourself in the foot, though, and everything must be configured properly to get to feature parity with GHA (mail server, plugins, credentials, sso, https, port forwarding, webhooks, GitHub app, ...).

But once those are out of the way, its the most flexible and fastest CI system I have ever used.

The Jenkins vitriol is also puzzling to me, I think the security model, reliability and backup/restore story has gotten seismically better in the intervening decade people wrote it off

Nah I don't mind Jenkins either. I think it's unpopular because you can definitely turn it into a monstrosity, and I think a lot of people have only seen it in that state.

I also like Jenkins. I think you can turn it into a mess, but in the right hands it’s a powerful tool.