The problem with soft-links is that if the underlying file/directory is deleted, you are screwed. For example if you have 100 full-machine backups and want to free some space so you decide to delete every other one, you have to be careful that none of the backups you are keeping, have soft-links to files in the backups you are deleting.

With hard-links the underlying data is not deleted until all hard-links are deleted, so you can delete any individual backup directory without losing data in any other backup directory.

A soft-link is like a pointer in C whereas a hard-link is like a C++ shared_ptr, ie. reference counted.