Spin waves, also known as magnons, are low-lying collective excitations in magnetic materials, for which it is hard to achieve agreement between first-principles electronic structure calculations and experiments. It has been shown in literature [I. Galanakis and E. Sasioglu, J. Mater. Sci. 47, 7668 (2012)] using as a prototype three full Heusler alloys-Pd₂MnSn, Ni₂MnSn, and Cu₂MnAl-that usual density-functional calculations for perfectly ordered compounds fail by a large margin to reproduce neutron scattering measurements of spin waves. We show for these three compounds that the inclusion of correlation effects in the form of the GGA+PU approach and/or substitutional disorder accounted via the coherent potential approximation affects considerably the calculated magnetic properties and their agreement to the experimental data. We expect our results to pave the way for further studies on magnetic materials for which experimental magnonic data exist.