Of course unibody vehicles have a frame, consisting of tubular members of high-strength steel welded into the unibody structure. Their strength varies depending on the purpose of the vehicle. The same could be said of body-on-frame vehicles. Some ladder frames are quite robust, and others not so much. Whether unibody or body-on-frame, manufacturer specs, including tow and payload ratings, factor in the ability of the frame to manage the loads.
In this photo you can see one of the rear frame rails on my unibody Pilot. I got a good look while the vehicle was being repaired after a rear end accident. It is quite robust, consistent with its maximum 4500/450# tow rating. There was no damage to the unibody structure, just the hatch, bumper, factory receiver, taillights, and spare.
Here's the vehicle that hit me.