Which of the following is a common feature of modern unibody designs?

Integrated safety systems are indeed a hallmark of modern unibody designs, reflecting the advancements in automotive engineering focused on occupant protection and crashworthiness. Unibody construction refers to the vehicle's body and frame being a single structure, which allows for better energy absorption during collisions. Many contemporary vehicles feature crumple zones and reinforced passenger compartments as integral parts of their design. These systems are built into the vehicle's structure, ensuring that safety is not an afterthought but an essential component from the ground up.

The other options, while aspects of vehicle design, do not characterize unibody construction in the same way. A separate chassis is more relevant to traditional body-on-frame vehicles, where the body and chassis are distinct. The complexity of the assembly process has evolved, but it doesn't specifically define unibody designs. Heavy steel construction is less common now, as many modern vehicles utilize lighter materials such as aluminum and high-strength steel to improve fuel efficiency while maintaining safety and performance.