At another end are things that you should only be implementing yourself in highly unusual circumstances, such as cryptography libraries.

Where on this spectrum do you usually implement the thing yourself, and what is your thought process?

What are the costs of importing a library, and how are those costs different for different projects? Are there security risks? What are the benefits?

Conversely, what are the costs and benefits of implementing something in-house?

Has there ever been a case where you've tried to roll your own and it ended in disaster? Or where you'd wished you'd never added a dependency on some library?

[1] https://www.theregister.co.uk/2016/03/23/npm_left_pad_chaos/