probably at least 95% of the code would be the same between the AU and VST versions.
which makes it even more annoying that they can't seem to track it down - if the problem is in the small amount of code that is different for AU and VST but also common to both PC and Mac.
Apple doesn't make life for developers easy. AU's feature regularly in Chris Randall's nightmares. He's one of the developers of Audio Damage who also runs the Analog Industries blog where you'll find regular rants about Apple's changes to the format.
It is a needless format, it started becoming messy and if it were to evolve differently from VST's, there would be more format exclusive plugins. Let it die. I'll miss Automat, but thats about it.