Psionics in Traveller are tools in the toolkit -- very useful, but subject to being countered, like all of the other tools. Only some 10%-20% of Zhodani forces on a table usually have any psionic ability, and most of them are just mid-level telepaths.
Some advantages:
-telepathic communications can't be jammed
-clairvoyant spotting can't be jammed (like electronics) or blocked (like vision)
-teleportation can cause problems for the other side ("where did those guys come from?!")
The Imperials have psionic shields to prevent their minds being read in combat; they take teleportation into account during tactical deployments; etc.
So no, my psionic troops are not especially deadly against same-level troops.
Forces without psionic shielding or psionics, however, are probably toast, and they probably won't even know why they were toast. "How did they know we were here? Is our operational security that weak?" No, dude, they either read your mind or spotted you using clairvoyance. Then called in an ortillery strike.
Albeit they made me do things voluntarily as well … 
So …
