PX4's lightweight SIH (Simulator in Hardware) SITL runs physics and sensors entirely inside PX4 — no Gazebo, no GPU, 30MB Docker image. It's fast and CI-friendly, but currently simulates an empty, flat world. This limits what flight behaviors can be tested.
Meanwhile, PX4's SITL test coverage has significant gaps in environment-dependent behaviors: terrain following, collision prevention, geofence recovery, GPS denial, and more. These gaps exist largely because the test infrastructure lacks a way to describe the environment the vehicle operates in.