Of course a lot of libraries are not thread safe. However, that's not at all rare, lots of libraries for other programming languages aren't thread safe either. My point is that those libraries won't start magically crashing when running in no-gil mode unless the dev using them starts using threads in Python. Yes, it's hard to know which libraries are thread-safe and which ones aren't, and just like any other language you should default to "not thread safe" unless the developer explicitly says otherwise or you inspect the code.