Incorrect Force models
The GUO force model implemented in waLBerla is only applicable for SRT collision models, since the terms which are applied are only correct for the SRT operator. Even for a TRT collision operator which is also applied in the population space, it is not correct to apply the force terms in the way it is done in waLBerla.
For reference, the book by Timm Krüger shows how to apply a GUO forcing scheme correctly for MRT methods in section 10.5.
Since it is rather complicated to apply the forcing for MRT models correctly in waLBerla directly, I would recommend deprecating it.