Additionally, do we know why they were said to be beneficial/detrimental? I think I remember a myth in some culture believing that eating your enemy's heart would give you their strength, but I'm not sure where I remember this from.
You're thinking about the Iroquois. There is a widespread belief, first started by missionaries to illustrate the savagery of the natives, that the Iroquois would torture and eat the hearts of their captives. As the stories go, Iroquois braves torture prisoners to see who can endure for a long time without fear, and then eat their hearts in order to inherit their victim's courage.
There is, in fact, no evidence for any of this.
However, there are many genuine myths about eating body parts. In general, it is believed that some kind of essence or property could be transferred via consumption. For instance, in Chinese mythology, Hsuan-tsang was hunted by monsters on his way to the West because eating his flesh is thought to grant immortality. The reason in this case was because the monk was the reincarnation of a sanctified soul.
Another common effect is that eating the heart of someone allowed their essence to be reborn. Examples include several variants of Dionysus's birth. In one, Zeus was said to have eaten the heart of Zagreus, the "first Dionysus", and thus impregnated the second incarnation in Semele. Likewise, in Norse mythology Loki is said to have created all the female monsters of the world because he "once became pregnant by eating the half-burnt-heart of a woman".
More generally, many cultures attributes specific properties to different body parts. The eating of these parts thus strengthens the associated properties. Eating the heart for courage; another is eating sexual organs for virility. This is also true beyond cannibalism. Norse myths offers several examples of eating the hearts of animals, such as when Bödvar Bjarki had Hott eat a heart to gain bravery. Chinese folk medicine to this day includes eating dried deer penises for virility.
 Arens, William. The Man-Eating Myth: Anthropology and Anthropophagy: Anthropology and Anthropophagy. Oxford University Press, USA, 1979.
 Auerbach, Loren, and Jacqueline Simpson. Sagas of the Norsemen: Viking and German Myth. Duncan Baird, 1997.