Cobalt nanoparticles are synthesized by chemical reduction in reverse micelles using 6 different products of NaBH₄ reducing agent. The nanoparticle size polydispersity strongly depends on the NaBH₄ used. With the same synthetic approach, only one product yields to uniform size and shape nanoparticles. This result highlights that the NaBH₄ plays a key role in the reproducibility of (1) the synthesis of Co nanoparticles characterized by a low size polydispersity, and (2) their 3D assembly. I stress that this finding can strongly affect not only the development of nanoparticles and their 3D assemblies but also our knowledge concerning the nucleation and growth mechanisms at both the atomic and nanoparticle scales, which can be biased, if we do not account this additional key parameter among all the ones involved in the colloidal synthesis.