As much as I'd LIKE to agree about the Mantis, I can't. Its only OP on situational basis. Every time I've gotten in the Mantis, and I do mean every time, I get ONE kill with it, before the thing is obliterated. Everybody totes Plasma Pistols and nades the hell out of it. Its essentially unusable.
I got way more kills splattering people with the Mongoose and the Warthog than I did running around with the Mantis.
Also, you did list that you killed vehicles. Its got a fast firing rocket launcher and a heavy machine gun that tears vehicles apart. Of course it killed the vehicles. However facing off against a sizable group of players who work together even a little bit, that thing is good as dead.