Overpowered? I have not seen a demon ship used in months. I always found them to be under powered. When they come into game, they come in at the START of the enemy phase, letting them get free shots off on a vessel that has not moved (being a defense is a bitch). Usually it ends up braced or crippled the moment it comes into play.
Also, the upgrade is WITHERINGLY expensive, especially on larger cruiser hulls where popping in behind your enemy would be the most advantageous (though a slaughter would do well). You also cannot take space marine crews.
If you got, say, the mark of chaos of your choice for free, then the chaos demonship would be FAR more useful.