  The main goal of this work is to compare the results of three dynamical mass
estimators to the X-ray hydrostatic values, focussing on massive galaxy
clusters at intermediate redshifts $z\sim0.3$. We estimated dynamical masses
with the virial theorem, the Jeans equation, and the caustic method using
wide-field VIMOS spectroscopy. We investigated the role of colour selection and
the impact of substructures on the dynamical estimators. The Jeans and caustic
methods give consistent results, whereas the virial theorem leads to masses
$\sim15\%$ larger. The Jeans, caustic, and virial masses are respectively
$\sim20\%$, $\sim30\%$, and $\sim50\%$ larger than the hydrostatic values.
Large scatters of $\gtrsim50\%$ are mainly due to the two outliers RXCJ0014 and
RXCJ1347; excluding the latter increases the mass ratios by $\sim10\%$, giving
a fractional mass bias significant at $\gtrsim2\sigma$. We found a correlation
between the dynamical-to-hydrostatic mass ratio and two substructure
indicators, suggesting a bias in the dynamical measurements. The velocity
dispersions of blue galaxies are $\sim15\%$ ($\sim25\%$ after removing the
substructures) larger than that of the red-sequence galaxies; using the latter
leads to dynamical masses $\sim10\%-15\%$ smaller. Discarding the galaxies part
of substructures reduces the masses by $\sim15\%$; the effect is larger for the
more massive clusters, owing to a higher level of substructures. After the
substructure analysis, the dynamical masses are in perfect agreement with the
hydrostatic values and the scatters around the mean ratios are divided by
$\sim2$.
