What do you mean when you say unsinkable, using the deck gun or torpedoes? The reason I ask is because I ran into a small subchaser and a Gyoraitei PT Boat and couldn't sink them with my deck gun either, even after a dozen hits on each, albeit with a 3" gun. I'm willing to guess that a larger gun might do the trick. Now, I simply sneak around them and avoid them altogether. I wouldn't think it's too easy to sink a Fubuki DD with a 3" deck gun either but, I sank one with 1 torpedo.
Don't get frustrated, all the mods take a little getting used to and have varying differences to their style of play. I'm playing RFB/RSRDC right now and am having fun, despite the unsinkable small subchaser.
OK, I just read your post in the "what are you up to" thread. I've never heard of an unsinkable Fubuki. Are you sure you had the correct mod, I mean the right RSRDC for the verson of RFB you were playing? It sounds like maybe something wasn't installed correctly or was having conflicts.