Borrowed code making testing less reliable?

I once inherited some code that uses 40 modules and libraries. These were software packages written by strangers that are posted mostly to Github. This project's code had never been tested in an automated way. All testing was done manually by cheap workers overseas and it was terribly buggy code. There was no plan to do automated testing when I arrived there and I was not allowed to make the case for automated testing.

Question: In what negative ways can the 40 modules and libraries impact the automated testing of the program ?

Fun fact, in my most recent job we did do extensive testing and everyone categorically refused all use of random modules and libraries because they were Unknowns and it was difficult to write tests to include them.