  We present an angle-resolved photoemission study of the electronic band
structure of the excitonic insulator Ta$_2$NiSe$_5$, as well as its evolution
upon Sulfur doping. Our experimental data show that while the excitonic
insulating phase is still preserved at a Sulfur-doping level of 25$\%$, such
phase is heavily suppressed when there is a substantial amount, $\sim$ 50$\%$,
of S-doping at liquid nitrogen temperatures. Moreover, our photon
energy-dependent measurements reveal a clear three dimensionality of the
electronic structure, both in Ta$_2$NiSe$_5$ and Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$
($x=0.25, 0.50$) compounds. This suggests a reduction of electrical and thermal
conductivities, which might make these compounds less suitable for electronic
transport applications.
