Agree.
Higher-level enemies inflect too much damage too quickly. A blaster may survive the first hit or two to get to the buff at 40%, but one more hit will end the fight.
Without significant other benefits, Defiance is not worth the risk, especially when the damage cap can be hit with aim + buildup + a few hundred influence for red inspirations.
For Defiance to be viable I hope the Devs would consider a bigger damage buff sooner, significant accuracy buff, reduced recharge time on attacks and reduced activation time on attacks. Consider starting the buffs at 50% HP with straight line scaling so that the damage cap is reached at 15% HP, 100% accuracy buff at 15% HP, base recharge time halved at 15% HP and activation time halved at 15% HP and auto conserve power.
Would this be overpowered? Maybe, but Defiance as it is doesnt have the any power applied yet.