No experience or comment on Python/Tornado. We don't really do a lot of web stuff.
But sure, people can screw things up in lots of ways. However, once a threaded program is screwed up, you really only fix it by starting a new version - it's near impossible to incrementally fix race conditions and dead locks - you can't reliably repeat the bug to debug it. Bugs in non-threaded code can at least be tracked down one by one.