There are several threads discussing Shadow Elves already. If you search for Shadow Elves I bet you can find threads about it.
I think the general opinion is that Shadow Elves are prone to get overpowered, and are far too easy to powergame with. BUT, if a serious player that wont powerplay as much, some say they actually work good.
My advice is to just try them out for a test game, then decide if they are overpowered.