Raft slabs are cast against the ground whereas waffle slabs are cast onto polystyrene void formers and strips of concrete. An overloaded raft slab is less likely to crack because it is cast onto the ground.
Why? Waffle slabs are built on top of the ground and need great drainage and really hard ground for the life of the building. Raft slab footings are dug into the ground, have more perimeter stiffness and more tolerance to poor ground conditions than waffle slabs.
When landscaping and gardens are placed around waffle slabs water can easily flow under the slab affecting the foundations.
Poor site drainage is easily overlooked. Even if the ground surface looks like it grades away from a waffle slab, builders often use sand and rocks around a house to bring up ground surface levels. Sand and rocks are porous and allow water to flow under a waffle slab. This is a major cause of slab heave in waffle slabs