Abstract: Recent experiments have claimed successful synthesis of cubic boron-carbonitride compounds BC2N with an extreme hardness second only to diamond. In the present work, we examine the ideal strength of cubic BC2N using first-principles calculations. Our results reveal that, despite the large elasticity parameters, the compositional anisotropy and strain dependent bonding character impose limitations on the strength. Consequently, the hardness of the optimal BC2N structure is predicted to be lower than that of cubic BN, the second hardest material known. The measured extreme hardness of BC2N nanocomposites is most likely due to the nanocrystalline size effect. These nanocomposites may point to a new method for the synthesis of new superhard materials.