  The charged pion multiplicity ratio in intermediate energy central heavy-ion
collisions has been proposed as a suitable observable to constrain the high
density dependence of the isovector part of the equation of state, with
contradicting results. Using an upgraded version of the T\"ubingen QMD
transport model, which allows the conservation of energy at a local or global
level by accounting for the potential energy of hadrons in two-body collisions
and leading thus to particle production threshold shifts, we demonstrate that
compatible constraints for the symmetry energy stiffness can be extracted from
pion multiplicity and elliptic flow observables. Nevertheless, pion
multiplicities are proven to be highly sensitive to the yet unknown isovector
part of the in-medium $\Delta$(1232) potential which hinders presently the
extraction of meaningful information on the high density dependence of the
symmetry energy. A solution to this problem together with the inclusion of
contributions presently neglected, such as in-medium pion potentials and
retardation effects, are needed for a final verdict on this topic.
