In theory, all of the options exposed in the UI on MC Mac should work. The vast majority of the application does work, and any features that do not, but which you can "see" from within the application, should be reported (which may be a bug, or reveal a problem with your setup).
I think you've found one here. I don't know for sure... I don't have a scanner hooked up to my Mac, and have never even tried the built-in cover art scanning functionality under Windows. But, I wouldn't be surprised to learn that the Mac version doesn't (yet) support scanning. It also doesn't support ripping (unless this changed very recently). Assuming I'm right and this shouldn't work, then that's a bug, though not in that it doesn't work, but in that you shouldn't be able to see the option in the UI. It got missed, in other words.
Generally, the Mac version of MC is a complete product. It does what it can do very well. But, it is not feature-equivalent to the Windows version (which has been under development for way more than 12 years). It will improve, though you can expect major new features to come with a major new release version (and so therefore, requiring a paid upgrade).
The rest should be hidden. There is certainly a lot to do to match the Windows version, but what is there is very good. If you have specific examples of where it isn't working, then lets figure out why and where.
