The most difficult case is when writing the code from scratch actually is a lot faster than choosing and learning the API for a library, but in the long term ta

The most difficult case is when writing the code from scratch actually is a lot faster than choosing and learning the API for a library, but in the long term takes more effort to maintain (higher TCO). The choice depends not only on how complex your own code would be, but also how mature and well maintained the library is (e.g. maybe a library that's just right is some solo developer's pet project, whereas a slightly more awkward fit is actively developed by a big company). That is the sort of thing where making the right choice really requires a lot of experience.