Where should particularly heavy equipment such as a UPS be positioned?

Low down where practical.

That isn’t because the bottom of the cabinet possesses some special cooling property. It’s primarily mechanical: keeping heavy equipment lower reduces the height of the combined centre of mass and generally improves stability.

Take the same heavy UPS and move it from the bottom of a freestanding cabinet to the highest available U. The UPS hasn’t changed at all, but you’ve deliberately moved a substantial mass further from the supporting surface and made the whole arrangement less forgiving if it’s disturbed.

The UPS still needs whatever mounting rails or rear support its manufacturer specifies.