Hi,
I want to answer to the question asked by a simple point:
If your psycker use his power to often you can not do anything truly without talking it out with him, but in game, using ghost form to enter the lair of a gang of brain damaged cultist will surely split the mind of the psycker. What is by your view the warp reflect of the hidden temple of a cult, the hiiden laboratory of a tech heretic or any strongpopint where "bad guys" stay for long…
My answer is HELL!
No sane psycker will expose himself to that taint and to the denizens of the warp that are called by such place. Maybe the problem you meet is more linked to your lake of description of some feelings. As a GM since 1992 i've learned some things and today none of my playder DH, DW or RT are going to use a ghost form in a place describe as sucking their essence, making their edge go hazy and giving them a good headhake.
The point is not to make the life of your player bad but just to put some color and smell to a virtual world who's different from mind to mind. If you want your player to understand things you have to make them clear. And then maybe your psycker will not risk is soul in a land of despair feeling his "bones" becoming cold as the grave and with nightmarish vision appearing from time to time.
And to come back to an other post, lots of people have CP even loyal servitors of the Emperor and in itself isn't a 100% chance to find "bad guys"
Btw make use yourself of a NPC with telepethy power using them to blur and make false the vision of your team, they will never more count it as 100% sure.
