It is completely balanced. The Mantis is good on only about half the map, that half being the side you control, once you moves beyond that safe limit all attention is focused on the Mantis, and if you stay on the side chances are you will die within a minute.
It is more of a support vehicle if you want to successfully use it as it can hurt people from far away and kill vehicles in a snap.
I truly believe that if you are assaulting a team in a Mantis, you are using it wrong.