I main the Role Tank and the problem I see when going shield tank is
a. My Shield Breaks faster than their Shield in essence making me useless.
b. No one uses my shield - so why do I shield tank?
Normally its the DPS who fails to see that my shield is useless if I have no damage behind it, which falls on the DPS responsibility - in these cases, its better to go Ball or any tank other than a shield tank such as Orisa, Rein, Sigma.