rimu talked about this a bit in a fireside fedi chat a while back (~24:00 for tech stack discussion and ~33:00 for python specifically).

As for async, all of the federation work happens asynchronously through the use of celery tasks. Federation work is actually a pretty sizable portion of the computational demand, so offloading that to workers in the background helps a ton. So, the only thing that flask is really responsible for in terms of serving pages is the web UI and the API. It might not scale to the size of reddit very well without a ton of work, but it has been fine for piefed.social. Just like lemmy (or honestly most other applications like this), the main bottleneck is the database rather than any kind of computational overhead for the python framework or rendering speed.

A big advantage of the stack is that it is relatively simple. If you know some python and some pretty standard html/bootstrap, then you can make meaningful contributions (I wrote the dev docs and I am not a professional developer for example). This has led to a large number of contributors that have worked on the features that they feel are important to them since the barrier to contribution is fairly low.

I think the primary disadvantage of using a framework like flask is that it led to PieFed being initially created as just a web interface without an API. Trying to tack an API onto the application after the fact has been a pretty heavy lift, and there are still areas where the API is lacking compared to the web interface. It is something that we have gotten better at now, we add things to the API at the same time as the web interface, but there is a decent backlog of features yet to be added to the API.