Weak Armor
Too Much Bloom On Regular Gun
Too Quick of a Overheat Time
Rockets are just useless even against vehicles since most heavy vehicles can take you down in a couple of shots.
The only mantis that is actually alright is the ONI mantis and those rarely drop.
I think just about everything in the game is pretty well balanced – but i would have to agree that the Mantis is in need of a buff.
One of the reasons it was good in Halo 4 was you could use the rockets in a much more precise and unpredictable matter. If i remember correctly, you also could not board it from the front.
I would be happy with the base version being the Halo 4 version, and the rest being buffed armor (because lets be honest, the base version of all the vehicles have paper armor).