The Unarchiver (available on the Mac App Store) is the Jack-of-All-Trades decompressor of choice for me on my Mac these days, and can actually handle quite a few versions of StuffIt archives.
> The source code of Keka 1.0 will not be public due some legal issues. Legal support is needed, if you can help the project, please get in contact on info@keka.io or the Project page on the official Keka website. Any help is welcome.
For that reason, these days I tend to stick with 7-Zip running through CrossOver for my archiving needs.