Of course it will work fine most days, QF would be insane to think about it at all if that wasnt the case. The problem is two fold
1) Any problem gets highlighted by media-worlds longest flight nearly runs out of fuel etc
2) The destination is LHR where on tiny hiccup can ruin a lot of folks day and a tiny hiccup at the end of 20 hour trip is more of a problem than if the flight started in Berlin or even JFK . LHR imposes its own brand of delays quite often and QF dont have much a a presence in Europe if they are going to divert .