The Holy Knights are too weak to be meaningful

I'll keep it a buck: The Holy Knights will either get no diffed or no diff, no in between. No interesting fights. Oda isn't killing meaningful characters, so the HKs aren't going to be a realistic threat to anybody unless Oda actually changes his ways. So you have characters that will have zero interesting fights and have guaranteed zero stakes in the fights they have the upper hand in.

They have no value to the plot aside from being filler villains to move things along and give other people someone to beat on (and not kill) whenever Luffy is fighting Imu.