The primary unsoundness is lack of types. The type checkers have strictness settings that will error if you are missing any types even types internal to a function/3rd party library used. The highest strictness settings what places are there that still are unsound? The very dynamic behavior that sometimes happens has two main impacts. Either type checker doesn't understand it and on high strictness settings will treat it as an error or you need to extend the type system use plugins (mypy has some plugins for very dynamic libraries to define custom rules).

Practically most large programs today can not pass maximal strictness settings as too many libraries lack type hints. But that's not a fundamental issue of python type system just a needed improvement of the ecosystem. Type hints are gradually growing and some major libraries have had noticeable improvements in the past year (numpy only started to have some types around 1.20).