  This note shows that in looking for exact solutions to nonlinear PDEs, the
direct method of functional separation of variables can, in certain cases, be
more effective than the method of differential constraints based on the
compatibility analysis of PDEs with a single constraint (invariant surface
condition). This fact is illustrated by examples of nonlinear
reaction-diffusion and convection-diffusion equations with variable
coefficients, nonlinear Klein--Gordon type equations, and hydrodynamic boundary
layer equations. A few new exact solutions are given.
