Recently, a novel process was presented to fabricate dense yttrium-doped barium zirconate electrolytes with high proton conductivity. This process was based on the use of a NiO additive during reactive sintering. We show here that materials made from this process present a fast degradation of mechanical properties when put in hydrogen-rich conditions, while material made from conventional sintering without NiO aid remains intact in the same conditions. The fast degradation of samples made from reactive sintering, leading to sample failure under highly compressive conditions, is due to the reduction of NiO nanoparticles at grain boundaries as shown from structural and chemical analyses using Transmission Electron Microscopy. By the present study, we alert about the potential risk of cell failure due to this mechanical degradation.