the deathwatch adversaries are not really underpowered, most of them are just right but need the appropriate situations to really be scary.
figthing 2 warriors in an open field isn't a problem, 2 warriors and a mag 30 horde of hormagaunts is.
the only enemy I really have any complaints about is the tau commander. he feels much more deserving of the elite classification than the master, as when my players fought him (in disadvantageous terrain to them) they managed to shred him down to -30 in less than a single round (krak missile x 1 RF, bolt pistol burst, double RFed stalker hellfire shot)
I've been toying with some profiles for the other nids in the codex that are not represented in deathwatch.
Genestealer (modified from the landsholm genestealer in final sanction)
WS / BS / St / T / AG / Int / Per / WP / Fel
65 / -/(12)60/ 60 / 60 / 30 / 55 / 40 /
12/24/36/72 move, wounds: 30
skills: awareness, climb, dodge +10, swim +10
traits: multiple arms, unnat str, Imp natural weapons (claws), dark sight, size (hulking), Natural armor (6), tyranid, brood telepathy
weapons: rending claws (2d10+12;pen5;special)
specials: stealth and rending claws
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Ravener
WS / BS / St / T / AG / Int / Per / WP / Fel
55 / 30 /(12)60/ (10)50 / 60 / 20 / 55 / 50 /
12/24/36/72 move, wounds: 50
skills: awareness, climb+10, dodge +10, swim +20
traits: multiple arms, unnat str, unnat toughness, Imp natural weapons (talons), dark sight, size (enormous), tyranid, fear(3), natural armor (8), instinctive behavior feed
Talents: Fearless, swift attack, lightning attack, rapid reaction
weapons: rending claws (2d10+12;pen5;special)
specials: stealth and biomorhps (range weapon in excg for lightning atk, toxic d10 )
let me know what you guys think!