Hardlink creation on directories would be unrevertable. Suppose we have :

- /dir1
 - this(.txt)
 - directory
 - have
 - files
 - and
 - directories/

I hardlink it to /dir2.

So /dir2 now also contains all these files and directories

What if I change my mind ? I can't just rmdir /dir2 (because it is non empty)

And if I recursively deletes in /dir2... it will be deleted from /dir1 too !

IMHO it's a largely sufficient reason to avoid this !