Twisted's problem is the good old singleton (anti-)pattern. There can only be one reactor ever, and most galling of all, it can never be restarted once stopped.
It should be said that Twisted has put a lot of effort into making it possible to making it possible to deal with multiple reactors (mostly so they can run their test suite including all their supported reactors), and even to make it possible to unit-test Twisted code without a reactor at all (by having standardized mocks for the various things the reactor does).
Of course, that doesn't help the mountains of code written for Twisted that expect a singleton reactor, so we're stuck with it for the foreseeable future. Perhaps in the Brave New World of Python 3, where Twisted is just an implementation detail of the asyncio module, life will be better.
