Most supplements that deliver quick energy -- and don't contain testosterone, EPO, cocaine or amphetamine -- use caffeine and sugar for that boost. Often it's guarana because it's a more concentrated form of caffeine and has a certain cachet that's often described as "natural" (as if coffee beans ain't natural?).
A quick Googling of various products named NRG, N-R-G, etc., indicates they all contain caffeine, which would account for the almost immediate and noticeable energy boost. A couple of cups of coffee or strong tea would have the same effect. But guarana has the advantage of not needing a pint or so of liquid to deliver that effect. And some folks just don't like the taste of coffee, black or green tea.
I couldn't find an NRG product that emphasized mushroom content, but only Googled for a few minutes. I love mushrooms and cook with them a lot. But they aren't superfoods. Most are pretty low on nutrients. Much as we mushroom fans love to tout their magical antioxidant and other properties, unless we're talking psilocybin mushrooms they're just funky looking chewable water with earthy flavors.
(BTW, there's a theory that the Viking "berserkers" were juiced up on amanita, that seems unlikely -- they'd be too sick to fight effectively. Ditto alcohol and other stuff. The most potent berserker potion for males since we climbed out of trees has been natural testosterone and tribal rituals to form bonds against a common foe.)
Chocolate is another popular ingredient for supplements that claim to boost energy. That's usually the caffeine content.
One of the NRG products also contains B vitamins, which can produce some noticeable effect in a few days for folks who were deficient in B vitamins. That may also account for some positive testimonials.
Even some popular gels touted for cyclists contain common ingredients for quick energy boosts -- caffeine and sugar, among other stuff. I tried gels for awhile to stave off bonks, but switched to cheaper glucose tablets made for diabetics. Helped on some 50 mile and longer rides, especially this summer when I couldn't stop to eat, or didn't want to feel sluggish from digesting a proper meal. During my most recent 50+ mile ride, Wednesday, I stopped to eat a huge BBQ lunch at the 30 mile mark and could hardly breathe for burping the next 23 miles. Sure tasted better than glucose tablets, though.