Context. A fundamental property determining the transient behaviour of core-collapse supernovae (CC SNe) is the amount of radioactive ⁵⁶Ni synthesised in the explosion. Using established methods, this is a relatively easy parameter to extract from observations. Aims. I provide a meta-analysis of all published ⁵⁶Ni masses for CC SNe. Methods. Collating a total of 258 literature ⁵⁶Ni masses, I compared distributions of the main CC SN types: SNe II, SNe IIb, SNe Ib, SNe Ic, and SNe IcBL. Results. Using these published values, I calculated a median ⁵⁶Ni mass of 0.032 M_⊙ for SNe II (N = 115), 0.102 M_⊙ for SNe IIb (N = 27), 0.163 M_⊙ for SNe Ib (N = 33), 0.155 M_⊙ for SNe Ic (N = 48), and 0.369 M_⊙ for SNe IcBL (N = 32). On average, stripped-enevelope SNe (SE-SNe: IIb, Ib, Ic, and Ic-BL) have much higher values than SNe II. These observed distributions are compared to those predicted from neutrino-driven explosion models. While the SN II distribution follows model predictions, the SE-SNe have a significant fraction of events with ⁵⁶Ni masses much higher than predicted. Conclusions. If the majority of published ⁵⁶Ni masses are to be believed, these results imply significant differences in the progenitor structures and/or explosion properties between SNe II and SE-SNe. However, such distinct progenitor and explosion properties are not currently favoured in the literature. Alternatively, the popular methods used to estimate ⁵⁶Ni masses for SE-SNe may not be accurate. Possible issues with these methods are discussed, as are the implications of true ⁵⁶Ni mass differences on progenitor properties of different CC SNe.