Even with Devourer, Xin Zhao still has strong dueling and ganks due to his kit.
What I meant was that Fervor of Battle gives him more time until his damage falls off without further offensive items. With it, I can quite easily purchase one or two defensive items after Devourer (often Deadman's Plate and Spirit Visage) before building Titanic Hydra without feeling like I lack damage.
I'm honestly torn between Grasp of the Undying and Fervor of Battle on Renekton.
On the one hand, Graph of the Undying is nice for a bit more burst damage and sustain when fighting and trading and Renekton typically has a lot of health. He can quite easily jump on enemies when it will apply to his next attack and force a trade in his favour.
On the other hand, Renekton autoattacks a lot between his abilities and his W applies on-hit effects 2/3 times. Fervor of Battle provides a lot of burst and sustained damage to his kit which can help him stay relevant even when he builds defensively.