There are a lot of issues that led to these fuels being abandoned the first time around, most of which are unavoidable. 1) Boron Carbide and Boron Nitride part

There are a lot of issues that led to these fuels being abandoned the first time around, most of which are unavoidable. 1) Boron Carbide and Boron Nitride particles form in the exhaust. These are extremely abrasive, destroying downstream engine components. They also make the exhaust plume highly detectable over long distances - think of the particles like soot from a coal burning ship. 2) The fuels are extremely corrosive, and thus you can't leave a tank full with them, and even when you do fuel up, you can't run on pure boranes, you need to mix them with other fuels. This means you can't keep a boron fueled missile on standby, so its speed advantage is rendered moot except for surprise first strikes. Further, in diluting the fuel the performance gains are reduced. 3) Boron based fuels suffer from incomplete combustion, dramatically reducing real performance below theoretical values. 4) Manufacturing and storing these chemicals is incredibly dangerous and expensive. One of the major reasons for the initial program's abandonment was that a production plant exploded. These safety concerns were damning in the age of atmospheric nuclear tests, one can imagine the difficulty of production in a modern regulatory environment. In short, boron based fuels offer only questionable performance gains at very high economic, technical, and human costs. I won't say there is no future at all for them, but the idea that they are the fuel of the future is laughable.