Summoning daemons in Black Crusade mostly involves rituals with specific requirements as to location, preparation, dark lore, etc. There is an Exalted (i.e. high-level Undivided) psychic power in Black Crusade that can summon in multiple lesser daemons for several rounds at a time.
Additionally, there are Perils of the Warp results that explicitly result in daemonic manifestation although the details vary somewhat between gamelines, including things like a Bloodletter popping out to assault the Psyker, mass daemonic possession of the psyker and everyone around him, possession of the psyker by a Greater Daemon or similar entity, and utter annihilation of the psyker resulting in a temporary warp rift that might spew out a daemon of commensurate power (and thus something quite perilous if the psyker had a high psy rating).
In the one OW game I've played in where we had a Psyker the Psyker dramatically dominated over all of the other characters beyond the first few sessions. He mostly just rushed the Warp Speed power and swift attack, and then poured whatever he could into Psy rating. The sheer number of hits he inflicted in melee via unnatural weapon skill made him significantly more effective than even the dedicated two weapon wielder melee specialist and his unnatural agility bonus ensured that he always won initiative by a mile and could dodge any blast effect (as well as almost always getting enough DOS's on a successful dodge to negate all auto fire multiple hits). He eventually grabbed Invisibility which allowed him to render himself virtually immune to ranged fire and make himself infinitely better at infiltration than everyone else. If he had not eventually lain hands on a Force Staff I'm sure he would've eventually coupled Warptime with Iron Arm or Flaming Weapon to boost his melee damage output.
Moreover, as things like Warp Speed, Invisibility, etc. are all sustains, he didn't even have any possibility of phenomena rolls except when initially casting these (which he did upon wakening) and when he used the Killing Will ability of his Force Weapon (which allowed him to one-shot a Necron Lord and just about anything else he encountered).
Later on, he still had opportunities to grab valuable utility powers like Gate of Infinity or Compel.
Of course, the GM was quite generous with experience and it would probably have taken a bit longer in a normal game to get the psy ratings necessary for this to be as overwhelming as it was, and the Psyker wasn't that potent very early on when he really only had Bio-lightning (although the real power of Bio-lightning lies in the opportunity it provides to make Palpatine references, something which the Psyker quickly caught onto!).
Additionally, Force Weapons are very hard to acquire and keep in Only War (but there are other psychic powers that can increase your melee damage by psy rating, as mentioned above- the only mechanical downside is the multiple sustains penalty to psy rating) normally. However, as with the other Only War campaign I've played in this was more or less an apocalyptic scenario where the actual Guard command structure on-world had collapsed and the player characters were left to fend for themselves or in charge of a small unit of survivors (and consequently weren't answerable to the Munitorum or anyone else).
Edited by Andkat, 10 August 2014 - 02:26 PM.