So I finally unlocked Mantis B after over a year of hearing everyone say how awesome it is. I loaded it up on hard mode... and died pretty quickly. Loaded it up on normal and was crippled early, limping along to the fifth sector. This was partly due to some really bad play (I chose going to a single unexplored beacon over reaching the exit before the fleet. I also lost a boarder early).
I tried again on normal. Not only did I win, but I purchased every upgrade possible. There was literally no more upgrades I could get without replacing a system. I had to dismiss 9 crew members. I gathered over 2400 scrap. I had a flagship killing weapon setup (albeit only barely) AND four max fighter skill mantis crew, a boarding drone, and a maxed out mind control. Not bad for my third try.
Still don't think its OP for hard mode. Its weak in the beginning* and has a hard time attacking the flagship**, both of which are bad news in hard mode.
*vulnerable to several fairly common events, has to buy a lot of systems before it can do normal things.
**hard mode flagship spawns with the missile and heavy laser systems connected to the helm, meaning you need a way to take out the medbay and to survive the flagship's weapons for a good amount of overall time. You also have to make a hard choice between cloak and MC because Mantis B starts with only 1 free system slot. Note that I don't use boarding against the flagship very often, so there may be an easy way I'm missing.