We seldom see what Tom does behind the scenes because he's basically a prop for Vito and Michael. I assume he was very good on the political end; otherwise Vito wouldn't have made him consigliere. Also seemed to have been quite competent in setting up Michael as a "legitimate" figure in the Nevada gaming milieu. I thought he was at his smoothest and most effective in setting up Geary and then taking advantage of Geary's terror by having him stay at Michael's home. I still believe Michael authorized the hooker trap earlier, but Tom carried it off brilliantly.

I'd like to summarize the screwups we see:

Tom couldn't be blamed for Vito's shooting. Paulie was Clemenza's responsibility. Nor was he at fault in Luca's death. Luca reported directly to Vito, who may not have told Tom about his mission. And anyway, it was Vito who put the garotte around Luca's neck by sending him to the Tattaglias with that idiotic story about "not being too happy with our family"--as if Vito would allow his "most valued friend" (movie), "a man who could do a job of murder all by himself" (novel) skip off and ply his trade for a rival family.

A charging bull elephant couldn't have deflected Sonny from his fatal final ride. But I believe Tom should have anticipated Carlo's treason. He should have known that the other families would be watching to see if the newest Corleone family member would get anything important--and when he didn't, to take note of that. And, after Sonny's public humiliation of Carlo, Tom should have seen that he'd be a prime candidate for treason during wartime. Overarching lust for vengeance may not be a typical German/Irish characteristic, but it's not unknown among associates of Mafia families.

Tom's failure to find out that Pentangeli had survived and was being held by the Senate committee, and his failure to learn that Questadt belonged to Roth, were inexcusably negligent.