The balancing present for the Mantis is actually very well done and interesting to look at. The Mantis is not a serious power vehicle unlike some people seem to think. That is why if you take your Mantis anywhere close to midfield on Ragnarok it will likely be killed in a few seconds.
Instead the Mantis is actually a support vehicle. With an arm mounted machine gun, semi-automatic rocket launcher, and deadly ground pound this may seem hard to imagine but it is the truth. The Mantis in all of it's full glory is reduced to a mere support role. Here's why.
~ Chainguns aren't accurate enough to reliably rack up kill unless in close distances.
~ When in close distances you are vulnerable to EMPs, Grenades, hijacking, and ordinance weapons.
~ Rockets Aren't quick enough to take out vehicles at long range with any sort of frequency.
~ Warthogs, Mongooses, Ghosts, and Banshees all have the mobility and agility to escape the Mantis.
So when you look at it the Mantis isn't really able to get in a position where it is able to rack up a good amount of kills without putting itself at a serious risk. The result of that is to just hang back with the Mantis and put damage on enemies wherever you can for teammates to rack up the easy assist, but mostly to be a prominent threat on the otherside of the hill (I assume we are talking about Ragnarok) to where the enemies cannot invade without taking the Mantis out first.