For quite a while I have found it a pain that unrar does not allow you to extract multiple files at once.
The other day I finally figured out how to get past this. Simply use xargs with place holders.

This command will unrar all the files in the current directory and any subdirectories to the directory /home/user/directory/

find . -iname "*.rar" | xargs -i unrar x {} /home/user/directory/