I had only one problem with emerald dragons - can't be ressurected
but on the other hand I haven't know about sacrifice real power
(but that would mean I would lose other units, and using to regain them I would have to use ressurection - to help me with ressurecting I could use inquisitors, but I had these inquisitors in my army and guess which army was attacked first by black/red dragons?... and my mana would probably be not sufficient for sacrifice + ressurection more than once per battle... )