1. The Klingons are based on the Soviet Union, are you going to argue that Soviet tech is good?
I've seen all sorts of claims of what the Klingons are based on.
Consider the Klingon Empire was clearly from the begining a warrior culture. Were the Soviets a warrior culture? There is very little of the Soviet Russian in them.
At most I see the U.S. - Russia cold war on the verge of flaring up to full scale war being emulated by the Federation - Klingon relationship.
I understand that at least some of the Russian tanks were excellent, but not handled well vs the Germans in WWII - the Germans had the training the Russians had the quality.
2. It requires a lot more training to be able to handle Klingon ships competently in battle. This should not be necessary for a purpose built warship.
Refer to above - warrior culture. Train early, train hard lose fewer in battle. The incompetent don't survive training or get demoted to infantry.
Don't forget Kor beat Kirk. The Organians had to rewrite the rules to give Kirk a draw.
3. The fact that fed ships which are built primarily as explorers can fight against Klingon purpose built warships demonstrates the inferiority of Klingon tech.
The Fed ships are multi purpose but the F-GCS is the TRUE explorer design. The F-CA while capable of surveying is not
primarily an explorer. It lacks long range probes and scout sensors. It is a warship that can perform exploration roles, but so can the D7.
4. Efficient warp design and Efficient combat design are two different things.
Its survival without a major over haul proves it to be well designed. As listed earlier the evolution from K-D7 to K-D7K upgraded weapons but only added 1 new weapons mount. The F-CA to F-CAR+ added 6 which show its design inadequacies, it didn't even have real rear defenses.