Often building structures on uneven or sloped ground, particularly the large footprint ones such as Heavy Vehicle Depots, can lead to very poor outcomes.

Quite often, existing plants/trees/rocks clip through the structures (Ideally these would be removed during construction)
On slopes, parts of the construction end up raised above the ground, floating in the air, or buried underground.
In extreme cases, it is possible to see underneath the structures.
Structures that should be flat by design, such as Helipads, often end up sloping.
Ideally large objects should either fold around the land better, split into smaller smaller individual items and clear the objects they are built on. They should also be flat as possible.