I am not sure if there would be cut and dry rules regarding this. The rule of the thumb for my startup is to always use libraries for major functionalities and

I am not sure if there would be cut and dry rules regarding this. The rule of the thumb for my startup is to always use libraries for major functionalities and for minor functionalities we either lookup the source and use the required function or we implement it ourselves. What works best for you might not be the correct answer for someone else. For us, we initially used libraries for all lat/lon and distance calculations and then refactored to move the functions to our internal code and get rid of the libraries. For algorithms like DBSCAN , even if it's straightforward, it didnt make sense to re-implement it. I guess it boiled down to the effort required to move inhouse vs the benefits