Nothing wrong with them but generally lore speaking you wouldn't see Grey Knights gallivanting around with regular Space Marine Chapters. Those that do come into contact tend to have their memories of the event forcibly erased. Only Chapter Masters know of their existence. I will say, their armor style is pretty 'effing awesome! Also, Grey Knights having a successor Chapter is just....wrong.
Grey Knights are too awesome for the initial release. You have to save some hype for later. In this case I hope it's much later... we need to build a huge player base first so we have enough players to fill all the factions. Once they bring out new factions you'll lose players in other factions... ect... I hope they don't even try anything with Inquisition or Grey Knights until they have a solid player base and the balance sway is figured out.
herewegoagain.jpg They don't fit into the lore here, no daemon primarch, no greater daemons corrupting entire sectors, and most of all, no way to equal them out with the other three factions
Let us see some official Q and A answer. http://forum.eternalcrusade.com/threads/mmorpg-com-dev-q-a.1163/ Q) Possibility of playing as a Grey Knight? A) David: Maybe, but not at launch.
DeathWatch would be a nice edition though I doubt it will happen, I wouldn't mind if they placed the black dragons, as the dragon claws tower over standard space marines, in the death of antagonis a black dragon' dragon claw stands face to face against a chaos terminator and he towers over the chaos terminator. Imagine infused protruding blades from both forearms I would like to see that!
1. I do not think you need to add the Grey Knights early in the game. I just wish them to be. 2. No need to add them as a separate faction. They can make up their own weaknesses: limited ammunition, a limited number of gray knights on the battlefield, and anything else