A friend was "burned" when starting to learn Python, and now sees the language as perhaps fatally flawed.
He was using a library and changed the value of an object's attribute (the class being in the library), but he used the wrong abbreviation for the attribute name.  It took him "forever" to figure out what was wrong.  His objection to Python is thus that it allows one to accidentally add attributes to an object.
Unit tests don't provide a solution to this.  One doesn't write unit tests against an API being used.  One may have a mock for the class, but the mock could have the same typo or incorrect assumption about the attribute name.
It's possible to use __setattr__() to guard against this, but (as far as I know), no one does.
The only thing I've been able to tell my friend is that after several years of writing Python code full-time,  I don't recall ever being burned by this.  What else can I tell him?
If the possibility to make mistakes is enough for him to consider a language "fatally flawed", I don't think you can convince him otherwise. The more you can do with a language, the more you can do wrong with the language. It's a caveat of flexibility—but that's true for any language.
Protection from accidentally misnaming object attributes in Python?