Right now, Deathwatch has two main advantages over Grey Knights:
1. Storm Shields. This is crucial. AP3, AP2, and Strength D shooting is at an all time high. While DW Librarians strangely cannot take Storm Shields, their Captains, Veterans, Vanguard Veterans, and Terminators can. For GK, only Draigo has a Storm Shield
2. The Psycannon was a pretty good all-around weapon until it switched to salvo. That hurt the Power Armored GK. Meanwhile, DW got the Fragcannon which is good against pretty much everything and is an assault weapon.
Deathwatch formations grant a nice boost but aren't game changers. Same could be said for the Nemesis Strike Force. So, they're pretty even their.
If GK get "assault from Deep Strike", that would be a game changer. Because GK feel like they really want to be an assault army with all of those Force weapons poking around.