The image-source method is the cleanest way to compute early reflections in a room with flat, parallel surfaces: mirror the source across each wall, and every mirror image becomes a phantom source whose straight-line distance to the listener gives you a real reflection’s delay and, with the wall’s absorption folded in, its level.
It is exact for a rectangular room with hard, flat surfaces, which is also exactly the kind of room that sounds harsh in real life. The moment a room has a curved wall, a coffered ceiling, or a diffusing surface, the method’s core assumption — that every reflection is a perfect mirror bounce — stops holding, and you need either a much more expensive ray-tracing model or a statistical, diffuse-field model for the later part of the response.
Our homepage simulator uses image-source for exactly the region it is honest about: the early, specular part of the reflection pattern in a simple rectangular room. Past that window it switches to a statistical decay matched to the room’s own Sabine reverberation time, which is the same split full auralization software makes for the same reason.