I have two (3000-cal) emergency ration bars in my truck, but for true emergencies, not hunting .I had exactly your thought, so as an experiment I ate one for a day or two. I won't do it again. You should buy and try one - they're pretty cheap on Amazon and then you can decide for yourself.
They're nutritious, but very bland, not super fun to eat more than once or twice, and not as light as you'd think. They're also intended to provide raw calories, not a balanced meal - most have little to no vitamins or fiber, and are just dense sugar cookies - fat and carbs. In a survival situation that's all you need - you're usually siting tight somewhere, or riding around in a life boat. You don't need much protein, and these bars are designed to keep you alive while help finds you, not vice versa. Hiking and hunting, you need the opposite.
Pemmican is great. But honestly, if you want a zero effort, fast, cheap, easy option, just bring store-bought jerky, granola, and M&M's. (I like to add raisins - you do you.) It doesn't need to be a $10-a-meal heavy-on-the-soy name brand entree. I mean, if you were willing to eat ration bars for 3 days anyway, it'll be no more monotonous, but also more cost-effective, nutritionally complete, and more satisfying.
If I'm "spiking out" I save a half pound of pack weight by skipping my kitchen entirely. I pack three snack-size ziplocs (lunch, dinner, lunch) with loose granola that I doctor up with M&Ms, raisins, craisins, and banana chips. It's dry so I need to bring extra water but it easily keeps me going for 48hrs, takes no time at all to set up, and costs like $2 per meal the way I roll.
