The first one is one of the dumber ones I've heard lately. It's bullshit.
As for the Arnold presses, they are no different from any other excercise in that if you do them improperly or if you use too much weight, yes, you will likely injure yourself. But as far as APs being dangerous per se, I think that's pretty much crap also.
However, APs are kind of an awkward movement. I would not recommend them to someone who had never done shoulder work before.