Neutrinoless double beta (0nbb) decay is a beyond Standard Model (BSM) process whose discovery would clarify if the lepton number is conserved, decide on the neutrinos character (are they Dirac or Majorana particles?) and give a hint on the scale of their absolute masses. Theoretically, the study of the 0 nbb decay involves the accurate computation of the nuclear matrix
elements (NME) and phase space factors (PSF), two key quantities entering the lifetime formulas of this process. In this paper I make first a short review on the actual challenges to calculate the NME and PSF for double-beta decay (DBD). Then, I discuss the influence that different nuclear approximations and parameters have on the computed values of these quantities, outlining the advantage to calculate simultaneously products of NME and PSF, instead of providing separately their values. This would help at a better interpretation of the DBD experimental data.