Double Iron Bash is a great move. The chances of you outspeeding are very low, so the flinching likely won't happen. But here are some reasons on why it is great:
1) 60 base power for a double hitting move means 120 base power, which is very high power.
3) It's actually a punching move, meaning that it gets boosted futher when melmetal has the Iron Fist ability.
Edit: Trick Room is extremely viable for Melmetal with Double Iron Bash.