[kylecordes]

Good: Libraries are leverage; as with financial leverage, use wisely to optimize results. At best, a library will be well tested, heavily reviewed, battle-proven code, and therefore worthy of trust.

Bad: Sometimes we convince ourselves that because a library has a large download count, it must be of high quality, and written by people far more qualified than ourselves. Sometimes this is true; sometimes a popular library is written by whoever was willing to write it first (and therefore grow adoption), which might not correlate with the other desirable properties.

Ugly: library authors naturally tend to be quite pro-library, more than the average developer. So they tend to bring in dependencies in unexpectedly large numbers. Your transitive dependency graph can grow unexpectedly large (especially in the npm ecosystem!). Your project can turn into the xkcd cartoon we are all thinking of, a tall tower built on some fragile bits you didn’t even know existed.