I would disagree. Cython is less known and has a learning curve. CPython extension interface is well established and more common. That is not an insignificant advantage.
Do you stop for a couple weeks to learn Cython, and do you have complete confidence in its generated code, or do you just start using something you know and is tried and true? It depends. We chose C Python extensions or just writing hotspots in C in a separate process.
The problem of C python extensions is that it is so hard to write correctly because of reference counting.