Apple's yearly Mac OS release is coming down the tracks, scheduled to crash into everybody sometime this fall. One of the great new features of El Capitan is a completely new "modern" version of the Core Image framework. Acorn uses the heck out of this framework, and does some interesting things with it.
Which is to say that there's some interesting bugs associated with Acorn and 10.11.
I've spent a good amount of time this summer finding bugs and reporting them to Apple when I can create a reproducible case. Apple has been good at fixing some of these as well, and I've been cleaning up places where Acorn was playing fast and loose with the Core Image in ways it probably shouldn't have been.
Acorn 5 shipped with a bunch of these fixes, and Acorn 4.5.6 is currently being worked on as well (we didn't forget about Acorn 4 just because 5 is out the door!).
So the current status is: there are a couple of known issues (like auto levels crashing), but both Acorn 4.5.6 and Acorn 5.0 are working pretty good on 10.11. However- if you encounter any bugs with it, we'd love to know about them. Let us know if you find any: email@example.com