  We show that the N-patch functional renormalization group (pFRG), a
theoretical method commonly applied for correlated electron systems, is unable
to implement consistently the matrix element interference arising from strong
momentum dependence in the Bloch state contents or the interaction vertices. We
show that such a deficit could lead to results incompatible with checkable
limits of weak and strong coupling. We propose that the pFRG could be improved
by a better account of momentum conservation.
