Almost every good enhancement used in competitive decks has some form of "Cannot be...", including a majority of the negates that actually see use. You cannot negate this card with one of those cards because of the restrict, and you can't play any other battle-winners either because they're all CB-.
It would seriously stop all of the major competitive offenses. That's too much.
My biggest problem with Sons of Men (the old version, I see that Asahel updated it) is that it is Brown. A 5/1 character that restricts the opponent from playing meta GEs is not really a problem unless it has EE backup. Well, right now EEs have never been more unimportant in the game, save for Plot, maybe Confusion, and chump-blocking EEs. If Sons of Men was Grey instead, it would be perfectly fine, IMHO.
Now I don't remember what it used to say. If it was also protected from cards that have "Cannot be" in the title, and that protection was CBN or CBI, then yes, it would be too powerful, even in Grey.
