geometry STBoundary returns the edge of a shape as another geometry instance. I inspect that result independently from the source. A polygon’s boundary is not another filled polygon with the same area.

geometry STIntersection returns the shared shape rather than only an intersection flag. I inspect its type and emptiness alongside its area. A shared edge has zero area but still contains points.

geometry STEquals compares the represented point set, which can stay the same when its text representation changes. I separate that question from byte-for-byte text identity. Reversing a line does not necessarily change the places it occupies.