I also agree with Lorenzo about the implicit trust concerning the autoupdate mechanism. Microsoft is notorious for dragging its feet in regards to even security updates which they justify by saying they have stringent quality controls, but as far back as Windows XP, I do not remember an occasion where auto updates crippled my machine or cause me any significant problems. In fact the only time I deal with autoupdates, is when I have a fresh windows install, which is the whole point autoupdates, you set and forget.
However, updating from Avast 6 to 7 has been a nightmare, realtime modules refusing to start, and registration problems, disruptions of popular programs like Chrome all of which in the end required me to use the specialized Avast removal tool and then clean install. Any one of these problems are a big inconvenience for a Avast enthusiast and technical users like me who have the know how and patience to work it through, for everybody else meaning the average users, they can be with some, not even aware and vulnerable if for example realtime modules refuse to start. extremely disruptive. Even if you lowball the estimate of 10 percent of users will inherit problems, that still means potentially millions and millions of users who will experience problem
Avast must understand that as a security vendor, it is held to a much higher standard that any other with exception of the OS itself; it isn't like itunes, where bonked update only means users can't listen to music. Users implicitly trust by their security software, to work properly, protect them and not cause disruptions which Avast 7 failed in all three. That's the whole point of security software. If you cannot trust your security software to update itself especially when it prompts you, then that reflects very poorly on the vendor to put it mildly.