if you really think about it, the attack difference isnt that high, moreso when rhydon has better stab moves. Better typing? Meh, its arguable as you said.
The gap in speed is probably the only thing that set them apart. Even though, UU is fast enough to deal with swords dance variants while ddance can´t really deal with bronzong/ferrothorn.
As an offensive player, I find haxorus isn´t that easy to set up against offense, so every calc I have seen in this thread is kinda pushing it too far.
Is it broken? Can´t tell, that´s why testing it for a while doesn´t hurt.