Overlords can spew creep underneath them. It's how you place creep/spore colonies or nydus canals in tactically useful areas without building a whole Hatchery.

I suppose, this versus depends on how competent the Orks are in tactical thinking.

fortifications can be bypassed using burrowed units like Roaches, Overlord drops, and Nydus Canals. Infestors can take out Squiggoths using a Neural Parasites. Poor aim does not bode well when fighting self-destructive Banelings or small and cheetah-like Zerglings.

You can take out a substantially larger Zerg force with adequate fortifications which are properly situated and protected with a few defenders. That is, if you know what to look for, and aren't expecting to repopulate your army faster than them.