Actually the interesting part in Angus is the Copper, a small "co-processor" (which can trace its roots to the Atari XL/XE series and its ANTIC processor) that could execute instructions triggered by the beam passing specific coordinates on screen.

The obvious advantage of this is that you do not need to interrupt the 68k CPU, which in the era of single digit Mhz speeds, was very nice.

The blitter (and other things) could be controlled by the Copper..

Yes, the Copper is a very cool thing. I've done insane copper lists with up to 52 splits per rasterline(each split is 8 pixels wide), and then a unique new set of colors for each rasterline. The result is very mesmerizing. It can look like this: https://youtu.be/d1axngYxuuo?t=3m19s and this: https://www.youtube.com/watch?v=fFeHd5hoRyM

The Copper can also wait for the Blitter to be ready, and the Copper can give the Blitter new instructions and start a new blit. The circle would be complete if the Blitter could blit into the hardware registers for the Copper and other chips, but it cannot. :-( I think the Atari STe Blitter can do that though.

But you can use the Blitter to modify the Copper list, so perhaps they together are Turing complete?

Yes, that is what I had in mind, but could not remember any more the details how it used to be.