Daz Bones' bounty was close to Crocodile's. A guy with an iron body, capable of weaponizing every inch of his body, turning them into blades, sawmills or other machinery, with Zoro having to unlock Haki to cut him down.
Other than that, yes, Luffy's opponents were stronger, but Zoro was known for obliterating his, while Luffy either had extreme diff battles or immense advantages (like vs. Enel).
There is a reason Zoro made it up there to the worst gen along with Luffy.