I was just reading something about NP-hard problems and cryptosystems. 

I was thinking: Every NP-complete problem can be reduced to another and every NP-complete problem has an equivalent (NP-hard) optimisation problem. A successful attack on one such NP-hard cryptosystem $A$ would mean that every other NP-hard cryptosystem $B$ would be vulnerable to that same attack; just reduce $B$ to $A$ and use the available attack. 

That would actually mean that we would be able to extend Information Set Decoding attack of Code-based systems to any NP-hard based cryptosystem.

Is this consideration correct?