Backlash for failed attempts is not quite OK, as at first you have a lot of failed attempts. This will kinda prevent you from attempting to psi-train your soldiers on live subjects.
But if you tie the backlash chance to the operator's skill somehow - low level PSI = less change of receiving a backlash for a failed attempt, and also reduced damage when the backlash happens - you may be able to balance it. I would go with a bit of stun damage as backlash.
Backlash from controlled enemies sounds logical, I would go with some stun damage and a little bit of morale loss for enemy units killed while being PSI controlled.
Also, why not make PSI attacks consume energy?