It's fair to say that Tom is not the best consigliere. He is loyal though. So perhaps some of his "screwups" with Michael specifically (Pentangeli's survival, the Roth-Questadt connection) aren't so much screw-ups as they are passive-aggressive games.

I mean Michael makes a big production out of letting everyone know "Tom's just my lawyer", "Tom please leave and close the door behind you", "Tom tell Al to join us", "Nobody discuss any Family business with Tom" that maybe out of hurt feelings/revenge Tom thought ok, you want me to be just a lawyer? Then that's all I will be. I will not do one iota of work more than what you've so graciously assigned to me..

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.