I haven't done a formal study but their t.co url shortener (which is forced on anyone who posts any url) is often broken or extremely slow. Twitter seems like a company where hardcore systems engineers probably take a backseat to "visionary product managers"

One of the best ways to resolve issues in a project is simplifying. You don't need so many if you are going to simplify. Though, anecdotally, I haven't had any issues with the site or service for a few years now. So I can't imagine they have many production issues that affect all the users.