Put most simply, a library needs to make my life easier, not harder.
Is it fully documented?
Is there a community of people already using it, with whom I can interact?
Does it scale up or does it fall over when stressed?
Is it cross-platform?
Is it easy to setup and get running?
If any of these questions have a negative answer, then that is a pretty major red flag. External libraries are only useful to the extent that they save you time and effort.