  Models with vector fields acquiring a non-vanishing vacuum expectation value
along one spatial direction have been proposed to sustain a prolonged stage of
anisotropic accelerated expansion. Such models have been used for realizations
of early time inflation, with a possible relation to the large scale CMB
anomalies, or of the late time dark energy. We show that, quite generally, the
concrete realizations proposed so far are plagued by instabilities (either
ghosts, or unstable growth of the linearized perturbations) which can be
ultimately related to the longitudinal vector polarization present in them.
Phenomenological results based on these models are therefore unreliable.
