Are dark energy models with variable EoS parameter $w$ compatible with
  the late inhomogeneous Universe?
  We study the late-time evolution of the Universe where dark energy (DE) is
presented by a barotropic fluid on top of cold dark matter (CDM). We also take
into account the radiation content of the Universe. Here by the late stage of
the evolution we refer to the epoch where CDM is already clustered into
inhomogeneously distributed discrete structures (galaxies, groups and clusters
of galaxies). Under this condition the mechanical approach is an adequate tool
to study the Universe deep inside the cell of uniformity. More precisely, we
study scalar perturbations of the FLRW metric due to inhomogeneities of CDM as
well as fluctuations of radiation and DE. For an arbitrary equation of state
for DE we obtain a system of equations for the scalar perturbations within the
mechanical approach. First, in the case of a constant DE equation of state
parameter $w$, we demonstrate that our method singles out the cosmological
constant as the only viable dark energy candidate. Then, we apply our approach
to variable equation of state parameters in the form of three different linear
parametrizations of $w$, e.g., the Chevallier-Polarski-Linder perfect fluid
model. We conclude that all these models are incompatible with the theory of
scalar perturbations in the late Universe.
