I must disagree.
Nids are my main 40k army, and while they have potent shooty units, you will probably need to get into a few assaults to win.
Good shooters -
Hive guard. They just rock. Great for demeching but can't do jack to infantry thanks to ap4 on their guns.
Flyrants with 2 devs. The majority of my lists include one. Hard to kill, very fast, offers defense against some flyers, and has a lot of power with guns. Can also assault too - he's an MC after all.
T-Fexes - Don't do rupture cannon. They sucked in 5th and are worse now. Use acid spray, cluster spines and a thorax swarm to lay do the templates.
Termagants - You should have tons of these, as you will include tervigons - probably the best unit we have right now, and they spawn termagants and require termagants to score. Alone he isn't scary. 20 aren't too threatening either. But 60 with 9d6 more per turn is.
Grax is right. We can't do all assault, but we never could. The best list is a mix. Stuff like trygons, hormogaunts, ymgarls, the swarmlord, warriors/shrikes, can all beat face in assault. MCs like the tervigon aren't assualt oriented either, but with claws put forth on average 5 s10 attacks (if you smash). Pretty vicious, but will just wreck vehicles.
So 6th hit us really hard with a metal baseball bat, but I have won a ton of alot more games than I've lost.
PS - we can take tons of psykers, and they rock with new powers and reduced psychic defenses.