People like you who are just mad about having been once on the recieving end, forget that MCH not only is slowed down when he tries to shoot you with Blastcharge, but also needs to ENGAGE IN MELEE with Bioblaster, to even unlock the Chainsaw in the first place. MCH in Melee range is very vulnerable and easy to take down, since all they can do, is run away or deploy Bishop-chan and hope noone enters the AoE out of fear. The Knockback effect from Scattergun can catch people offguard sometimes near a cliff, but the effect itself, only buys you arround a single second of time aganist melee Jobs and is outside of damage reasons useless aganist ranged jobs. Without the LB and the meek 3% instant death chance, MCH would have a incredibly hard time to land ANY Kills.
Stun and LB aside, NIN hits like a wet Noodle. In a open fight, even a BLM can easily overpower one in a 1vs1 scenario. Also, keep in mind that the Games netcode itself, is the NIN's biggest Enemy, because if they jump into stealth while you cast any spell or projectile (and they are technically still in range), the attack will knock them out of it, just like any DoT applied to them. This can severely cripple down most NIN Players.