" A result of 91 or higher always indicates failure, regardless of any other factors. "

I know it's been that way since Rogue Trader, but I feel it is a bit harsh. Thats a flat 10% chance of failure, no matter how skilled you are and how simple the power is to manifest.

Really?

You'd think a Psyker could improve his control of the Warp somehow yes, and use the basic techniques reliably?

Talking about "basic techniques", there are no Minor Powers now. Even Only War had something about using minor effects.

Lazy answer is, because the warp is fickle.

Being more helpful though, I think it was an attempt to have some chance of failure given how quickly psykers can get rolls of 100+ for powers in the old system (the additional modifiers to the power rolls they added post Deathwatch did help with this).

Personally I think this rule for failure should be removed at the same time as the +5 per psy rating (virtually all powers get better with rating, why make it easier at the same time).

Even better, do this and bring back the lower your psy rating/get bonuses on your power roll ability from the original beta to improve the risk/reward balance of psykers.