Santino was a good #2 or even #3 man but he could not think strategically and when his temper was up his rationality went out the window.

Vito thought that the ability to keep cool and think long term were the most critical abilities for a leader to have. So that's why he would have thought Sonny a bad Don.

"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.