Usually, polyfills extend or override global JavaScript prototypes and/or built-ins, affecting any code running in that environment. This may become especially problematic once a polyfill is not 100% compliant to its specification (which sometimes simply is impossible) and thus causing inconsistencies and bugs that are hard to analyze and debug.
For example, object-assign is a ponyfill for the Object.assign() method which was added as part of ES2015 and needs to be provided for ES5 browsers/engines.
To use the ponyfill, you’d have to import it (e.g. via const objectAssign = require('object-assign')) and then use it. It doesn’t override the prototype of Object and would not be available out of the box (as used to from polyfills).