The way it looked doesn't always correlate stronger attack. If thats true then Law Counter Shock is weaker than what we've shown. Which imo not true, especially since that also makes Doffy weaker in the first place.
This attack basically the same as how King Suic*de bomb on Zoro, bigger attack but did nothing (despite Zoro claim to clad his body into Haki) . That attack even weaker than Karyudon (who imo if Zoro's body got clad in haki would still hurt than tanked it with Body haki) .
And imo its not like Imu didn't do zero efforts. The way i see it is the same with G5, only limited time hence why they use their all out attacks. And after the time limit they get their drawbacks.
Kinda like how forbidden transform in all anime.