> I can't imagine any 2 pin plug being as mechanically stable as a 3 pin.
In absolute terms, yes. But in 30 years I have never seen a 2-pins europlug fall from a socket on its own. I still prefer the French plugs with the Earth pin (and I quite like the British one as well), but europlugs are fine and really not comparable to American plugs.
I can see it happening on some very old and very used ungrounded sockets. But with modern recessed sockets yeah not going to be issue. Which actually is an other great design of those plugs.