I would not object to the discovery of a hostile alien race which has a weapon that can shred our ships. Nor would I object to a weapon that is next to useless against ourselves but effective against a particular alien shield.
|Banu tachyon cannon|
More to the question is why would one think we should be able to perfectly adapt an alien technology? Geez, even poor little North Korea is having a bit of trouble developing ICBMs and nuclear warheads despite the tech being relatively well known.
Imbalance is not a problem but a challenge. It is the aliens that present the problem.
It may be reasonable to have perfect rock, paper, scissors, lizard, Spock within the UEE-developed technology. One would think the UEE knows the tactics of both deploying and defeating its own military hardware. But not so against alien races. A little desperation and panic on our part makes for good science fiction.