[Timothee]

Fair enough in general, but I don't think my conclusion of "Twilio's API runs on Flask => Flask is very reliable" is too far-fetched in the context of Twilio, whose whole business is their API.

To be fair to your skepticism, I have probably followed Twilio more than you've had. I seem to remember that they switched to Flask, so I don't think it was technical debt that they had to work around later on. (like it might have been the case with Facebook and PHP)

Let's say that "Twilio uses Flask for their API" answers the question "can it be used in production?" (or "is it a serious project today?"). "Should you use it in production?" is where context matters more.