Thanks for the information. But, just to be sure I understand. Are you saying the rhombic typically has 14dBd more gain than a loop antenna? But, the loop may still be better suited for DFing because of the nulls on each side?
A loop is generally better for DF because it has a very even radiation pattern, slowly rolling off from peak in a predictable way, and is deaf only end-on - gain is slightly better than a simple dipole. A rhombic has insane gain, but is extremely directional and basically deaf outside of its narrow forward lobe, and the secondary lobes off at 90 degrees to either side. That of course makes it excellent for fixed installations, and its ability to fold up very small for UHF and upper VHF is another large benefit.
Rhombics fell out of favor mostly because of how large they need to be when set up - four sides of 1.5-2 wavelengths is pretty beefy even on the 2M band, and ideally half a wavelength above ground. Not a problem for UHF, but big trouble once you start getting towards lower VHF, such as 6m, where each leg is 30 feet long at minimum, and about 10 feet off the ground, and for 160m HF you need 700 foot long sides and it to be about 270 feet in the air ... not ideal. Once the transoceanic cables opened to replace the HF radio links, they've mostly been a forgotten technology - Yagis are slightly less gain, but you can turn them on a rotator, even for HF bands, and height above the ground is much less important. Some amateur operators with enough space and finances still do use them though, since they can get around the world on the 20, 40, and 80 meter bands even when they're closed like they are now.