We report on the magnetization reversal in exchange-biased [PtCo]3 t PtIrMn multilayers with different Pt insertion layer thicknesses (0≤t≤1.2 nm). For t=0 nm and t=0.2-0.8 nm, magnetization reversal is asymmetric and proceeds by domain nucleation followed by a fast domain wall motion. For t=0.1 nm, domain nucleation is predominant. We interpret these results within a model for thermally activated reversal where a dispersion of the activation energy barrier is specifically taken into account. From magnetization relaxation curves, we are able to measure the barrier dispersion and identify the physical origin of different reversal effects: whereas reversal asymmetry is mostly due to local fluctuations of the anisotropy axis and exchange bias direction, the nucleation of a large number of inverse domains is caused by lateral variations of the interface exchange coupling energy. Moreover, we show that an improved perpendicular spin alignment in the outermost Co film maximizes the exchange coupling energy for a Pt insertion layer of 0.1 nm.