I agree with you, Imu should take down Luffy and Loki, but if he is not dominating Loki in his human form and in his zoan, what is he supposed to do vs Hybrid Loki and Luffy at the same time? Also yes Imu dominating Loki would also be disservice, but that part is on Oda.
But that is where the issue lies, his DF. I seriously can‘t understand the logic, on how that fruit is supposed to be a counter to Imu.
The Yami Yami No Mi and Nika makes simply more sense