I agree. I don't think its so much that Tom wasn't a wartime consigliere, I think it was more that Sonny just didn't have the tactfulness and quiet ferocity and yet gentle dipomacy that his father had. After all, it was Sonny that shot off his mouth at the meeting, not Tom. How is Tom a bad consigliere when Sonny does irrational things?

Plus, we never see Don Corleone giving Sonny tutelage. We see the Don and Michael after Michael takes over towards the end of GF1, and that's it. So maybe the Don and Sonny never had "the talk" as to how to be a Don,hence Sonny's distraught behavior.