The Scimitars were perfectly fine fighters--when they were first built. However, they are pretty much considered obsolete by WC1, and they are on their last legs in UE, only being used because of the Border Worlds' policy of using a fighter for as long as it will fly. Basically, it's as if current militaries on Earth were using WWII-era fighters--they were cutting edge when they were created, but have since been surpassed by a couple of generations of better ones.