Liver, kidneys, heart (technically a muscle), a lot cheaper than standard meat cuts now that I’ve found a *shop that sells them. But will I be getting more nutritonal elements, and less fat?
(*also available: uterises, testes & pizzles; but I’m waiting for the next office potluck for those)
I can only talk about heart meat.
There’s some fat on the outside of the heart. However, there’s no marbling within the heart. So, the meat is exceptionally lean, once you’ve trimmed the external fat off the meat. AIUI heart meat is also higher in calcium than normal flesh.
Be aware that getting a whole beef heart is a pretty large chunk of meat. I think the last one I got was 6 pounds or so. I recommend cutting it into strips for stir fry, myself. And marinating the meat beforehand. It can be very tough if you don’t do something to treat that before cooking.