alt Hacker NewsI run Jenkins (have done so at multiple jobs) and it's totally fine. Jenkins, like other super customizable systems, is as reliable or crappy as you make it. It's decent out of the box, but if you load it down with a billion plugins and whatnot then yeah it's going to be a nightmare to maintain. It all comes down to whether you've done a good job setting it up, IMO.
Lots of systems are "fine" until they aren't. As you pointed out, Jenkins being super-customizable means it isn't strongly opinionated, and there is plenty of opportunity for a well-meaning developer to add several foot-guns, doing some simple point and click in the GUI. Or the worst case scenario: cleaning up someone elses' Jenkins mess after they leave the company.
Contrast with a declarative system like github actions: "I would like an immutable environment like this, and then perform X actions and send the logs/report back to the centralized single pane of glass in github". Google's "cloud run" product is pretty good in this regard as well. Sure, developers can add foot guns to your GHA/Cloud Run workflow, but since it is inherently git-tracked, you can simply revert those atomically.
I used Jenkins for 5-7 years across several jobs and I don't miss it at all.