I think Jenkins is a very powerful tool but I would strongly disagree that it's quite valuable in a startup environment. Jenkins has struck me as a philosophical sibling as C++ - it offers a plethora of foot guns to the user that can also be used to accomplish rather good things. I would advise Jenkins to companies with projects that don't require agile responses to user requirements where the functionality desired at the end is very clear from the beginning - since the times I see Jenkins fail the most are when the configuration of jobs on it are being updated and reconfigured rapidly.

Jenkins tends to suffer from extremely poor state management and experimenting with things can cause a permanent loss of value to companies if backups are not properly configured - compared with a system like ansible where recipes will grow and be regularly committed to a repository Jenkins doesn't have a nature version control system and as a result strikes me as extremely brittle.

I almost wish the UI for jenkins simply didn't exist and it actually was just a whole mess of config files and shell scripts so that it could be locked down to a much better degree.