Look at nature. A lobster has an exoskeletal structure (shell is both skin and structure) while a mouse has an internal structure, where the skin has little structural value.
The lobster will not survive a long fall onto a hard surface without cracking its skin, but the mouse might survive from that same distance.
The mouse will not survive a certain steady weight placed on top of it, but the lobster might.
Each structure has its own benefits, and you can't strictly say that one is stronger than another.
If your environment involves a lot of falls and dropping, it's possible for one style of laptop architecture to survive better than another. If it involves being placed in a bookbag with 40 pounds of books on top, it might survive better with one architecture than another.
Neither base structure is better, stronger, or worse in every environment. The designers can, of course, take a little from both (and other) structure basics, and attempt to find a moderate solution that provides enough strength for all scenarios they believe their products will be subject to, but they all involve tradeoffs.
The unibody design is the strength of the macbook line. Being machined from a solid billet of aluminum means you get better flexibility than cast aluminum (ie, less brittle), and the fact that the skin is thicker than the skin on most laptops lends additional strength.
However, Apple does this primarily for looks, and they have made many tradeoffs to increase the laptop's appeal. For instance, the screen has very little protection, and the aluminum flexes enough that the screen may break under the wrong impact. Laptops with a different structure can protect the screen better. Not because the unibody design is inherently weaker, but the way Apple implemented that design to increase appeal at the cost of robustness means that Apple's overall design is weaker for that particular use case or environment.