User blog comment:DirgeOfCerberus111/Adeptus Custodes Vs. Grey Knights/@comment-8395098-20150327012537/@comment-8464494-20150328043434

Well some people have already done some GK squads but I don't figure this is an issue. You cannot make a chapters worth of those dudes, just one squad, so the chance of them being OP is minimal. And besides, everyone knows that the GK would probably beat the Custodes. No Custodes are known to have psy powers, every single GK is a psychic powerhouse to some extent. GK have been fighting for eons and eons, whereas I think the Custodes have only fought during the GC and perhaps the giant rift under the Palace, which may still be a thing. If thats the case its possible that the Custodes are even better at slaying daemons than GK, but I'm not sure if that rift is still ongoing since the Emperor was interred on the throne. Regardless, the fundimental difference is how they fight, GK are in the end, Space Marines, they fight as one in a pack style. Meanwhile Custodes are depicted as fighting apart from one another, like Lions as opposed to Wolves. It is reasonable then, that they could be divided and conqured in combat and that their battle lines lack some form of cohesion whereas the Grey Knights hold an actual formation and support one another. This means that Custodes, while certainly capable warriors, are not of the training that befits actual combat. Their good at taking out direct threats, but a protracted, measured assault combined witht the Grey Knights psychic powers might be a bit much for them to handle.