The targeting for the brain burst on psyker is very unreliable. I can be aimed dead centre at someone and it’ll go for another enemy slightly further forward but further left or right.
Its really frustrating.
Also tiny pieces of terrain can entirely block the psyker from targeting an enemy, most annoyingly when enemies are stood behind a railing that you can clearly see through. A veteran can easily shoot through that gap and hit them, so why can’t a psyker aim through it with their brain burst?