I was thinking when Vito makes Tom Hagen consigliere that he was showing possibly he was taking the first step towards making the family fully legit. He probably knew Tom would advise Sonny on coming up with less violent solutions to the family's challenges. He knew Sonny would be the Don as that was his destiny, but he didn't want that life for any of his children. To answer this question, I do feel that Tom as consigliere to Vito was a liability. Tom would have acted on his instinct and convinced Vito that the threat in Sollozzo and the other families was real, when Vito refused to get in the drug business. Tom failed to protect Sonny, as in the novel I think they mention how Genco would have sensed that the other families were going to use Sonny weakness (temper) against him.