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The "unlink" command can delete up to one file at a time, whereas the "rm" file can delete multiple files at once.

As far as I know, the "unlink" command uses fewer syscalls so it is better for performance. Also, "unlink" doesn't refuse to delete read-only files (chmod -w readonlyfile) whereas "rm" asks first.

Are there any other reasons I am not aware of?

https://www.gnu.org/software/coreuti...ink-invocation
unlink is the low level system call.
The unlink command will just call it. the rm command has a few options to make it more convenient, at the end it will call the same unlink function.

AFAIK, the only situation where you would actually need to use this command is when you have created "hard links." This is the case where more-than-one directory entry refers to the same underlying file ("inode"). And now you want to remove that "additional link," without necessarily removing the entity.

Also AFAIK, the much-more-flexible "symbolic link" has replaced this feature in common practice. It is: "a tiny file containing the name of another file."

But surely deleting the name wouldn't affect the content of the file if there was another filename hard-linked to it. As I understand it, delete reduces the link count of the inode by one and then checks if it is zero now. If so, it goes on to delete the file content. If not, it bails out at that point. A file in use would not have its content deleted and nor would one still hard-linked to a different name.

No, removing a link is all that the rm command does. Actually removing a file (i.e., freeing the storage) is something that the kernel does when the last hard link to a file is removed and the file is not currenly held open by some process. Using unlink instead of rm just bypasses some warnings that the rm command would give for things like files for which you do not have write permission.
Use of symbolic links is absolutely unrelated to rm vs. unlink. Both rm and unlink treat symbolic links exactly the same as the would treat anything else. When asked to operate on a symbolic link, neither one would affect the target of the symbolic link.

again, unlink uses the low level system call unlink, and rm uses the underlying low level system call unlink.
The difference is only the possibilities provided by the available command line options.