[@Necroes] Cool.
I didn't know your ork was an Alpha level psyker.

Because yes, psykers can manifest power, very powerful power in matter of moments.
But the more powerful the ability, the more powerful the psyker needs to be.

And i for one thinks the "psykers can cast several spells per turn" in the tabletop is a weird new idea they have. Me being a player from the old days of 3-5th edition tabletop.
But the tabletop turn is also a lot longer than a roleplay turn.
And a roleplay turn is a bit complicated to emulate in a play-by-post, since we don't actually use turns.

I'd say the fantasy flight roleplaying game systems would be better to compare to.
There, a turn is usually 1 action, which can be split into 2 half actions.
Attacking with a weapon is usually 1 half action. Aiming is another.

A full action can be firing your weapon on full auto, or reloading a weapon.
So a turn is roughly a couple of seconds. Unlike the tabletop where it is usually 6+ seconds, in the very least.

Psykers, can manifest several powers a turn in the rpg, they can. But not several attacks, and it would have to be quick simple powers.
The demolisher cannon-esque power, sounds a bit tamer than the "Smite" power from deathwatch.
That one takes 1 full action.
People have time to react to that. Hell, people can react to half actions. With dodges and parries.
They only get one dodge a turn, unless they have special talents.

So.
1: Most combat powers, especially powerful ones. Is clearly visible even for non-psykers. Classic things like the eyes burning with warp-fire, electrical bolts or auras are common with powerful manifestations.
Your argument holds true with low key powers. Like sensing the future, or manipulating smaller objects with thought.
But frikkin demolisher cannon levels? Definitely visible.

2: Urgrugg has actually not manifested many powers compared to what he could be doing. I agree with you here.

3: No, cover will help. However, depending on the power the power might be able to go through cover. But the aforementioned demolisher -esque power sounds like a bolt of energy.
If it's not, well, argument 1 still stands.

4: Yes there exist times where the the power backlashes, this ranges from COMMON to UNCOMMON depending on the level of control your psyker has and the stability of the veil of the warp.
Experienced psykers usually have little issues with them since they are powerful psykers, and powerful psykers can manifest simpler powers more easily since they don't have to strain themselves to do it unlike noob psykers.
Casting a lasgun bolt equivalent power is easy peasy for a powerful psyker, while it is not for a newly sanctioned one.
But more importantly, since the veil is weaker on this hulk, there is an adversely LARGER risk of ALL psykers to suffer from unexpected phenomena and backlashes.
Urgrugg being an unsanctioned psyker also doesn't have the same training as a sanctioned psyker, and thus will manifest phenomena more frequently than sanctioned ones.
I could go the next step to waaagh energy, but Urgrugg doesn't use that.

So seeing that Urgrugg is a powerful psyker, began as an ork weirdboy and with no sanctioning and using only experience to avoid phenomena, i'd say psychic phenomena should occur a bloody lot around Urgrugg.


But in the end, remember.
There are two named novel space marines in that corridor, one is a tech-marine, known for their resilience.
You are a big nob with no armour and a glowy stick. You are at the disadvantage here.
And you could have taken cover, seeing Urgrugg is described as thinking more humanlike, i'm honestly surprised he charged in.

I was gonna sit back around a corner, but i was apparently in line of sight from a bolter.
Unless the two marines are not beside each other, then the power won't hit both of them anyway.

And, this is a hulk.
Not all parts of the hulk is from imperial warships. Some parts are a lot less structurally sound.
So big booms might actually collapse large sections of floor, rip open walls to unsealed areas or just open up a door so a bunch of creepy crawlies come out.