Imu is exactly as I described: crap. Whether that can change, I don't know, but at the moment, Imu is crap.
Again, there is nothing crap about him at the moment.
Do we know the circumstances why Imu didn't kill Doflamingo? Because with one attack, Sabo was on life support. So Imu would have no issues doing the same to Mingo. Plus, he knows about the national treasure and blackmailed the WG. Even now, the WG sends assassins on Doffy's ass.
Sure, Imu could go and wreck Ohara all by himself but again, dude is just acting from the shadows and he doesn't show his presence to the world - canonically.
The WG is the constant winner who wrote history. Imu, as the head of the organisation, might have even lived since the Void Century, possibly killing NIKA. There is so much more than we readers know of.