Well, I wouldn't rely on The Unarchiver extracting sit archives correctly. The problem is not with The Unarchiver, but with underlying library it uses, libxad. This library has got some obvious issues with sit archives.

I tried to use libxad for my own purposes. The sit archives I've used to test the library were made with StuffIt Deluxe 10. In many cases, libxad didn't recognised names of the files inside an archive correctly. As a matter of fact, the names were correct, but not full paths. That way, some files and folders ended up in completely wrong places in archive hierarchy, often not even in the same hierarchy level. I could recognise that since I knew what archive I created, but the user trying to extract sit archive with unknown contents is very likely to get messed-up hierarchy of extracted files. In case of app bundles, plug-ins and alikes, this would make them useless.

Another example is archive made with StuffIt Deluxe, all files recognised properly by the libxad library. But then, adding a new file in the root of archive (using StuffIt) results in the library being able to see only that new file, all others (couple of hundreds of files become inaccessible! Deleting newly added file brought everything back to normal concerning library's ability to correctly see all files.

Problems also exist with sit archives containing encrypted files. If only one file inside an archive is encrypted, the library most certainly won't see all files inside. And an attempt to extract those encrypted that are accessible results in library function ending with data corruption error.

So, as a conclusion: I don't know for other archive types, but as for sit, the libxad library, and that also means The Unarchiver, is (unfortunately) not reliable at all. If anyone still needs sit support, I'm afraid there's no escape from StuffIt Expander yet.