Nope. It's powerful but not overpowered. It's the same as finding a strong weapon like the rocket launcher. It serves a purpose as a power vehicle, but it's not invincible.
I've done really well with the Mantis by hanging back and trying to focus on picking off vehicles more than players on the ground. If you rush up with that thing, you get two or three kills, then you're toast, you've wasted a great vehicle, and your team hates you.
Also, Banshees can make really quick work of those things.