There are circumstances where they are equivalent, but they're very different overall. Namely, if you use a service, you update it once and see the new behavior

There are circumstances where they are equivalent, but they're very different overall. Namely, if you use a service, you update it once and see the new behavior everywhere. If you use a shared library, you have to update and redeploy every service. Libraries are strictly inferior in that scenario. This sounded, to me, like it was Segment's problem. They were updating shared libraries all over the place all the time. I generally avoid creating shared libraries, they're a trap. They have a very narrow band of usefulness squeezed in between the more palatable solutions of creating new services or just copy & pasting code and allowing it to diverge for each different use-case.