There’s nothing inherently wrong with crunches. Let me start by saying that.
I don’t take a black/white, good/bad, do/never do, approach when it comes to movements. I like to approach exercises the same way I approach food.
There’s nothing inherently BAD about cheesy poofs. They’re just cheesy poofs. Eating cheesy poofs isn’t necessarily bad.
A better question to ask is this:
Will cheesy poofs get me to my goals? Are cheesy poofs an effective addition to my health system? Same goes with choosing what exercises to do.
If your goal/system (I prefer systems to goals) is a rockin’ bod that feels amazing, then cheesy poofs might not work within that system. If you’re trying to bomb-proof your core, then crunches might be a colossal waste of time.
Your best bet - especially if you have a tweaky back - is to become best buds with the plank and the hollow hold. I should also give a shout-out to hanging leg raises. These movements challenge all the muscles of your core. Crunches don’t.
Planks, hollows, and hanging leg raises are killer movements that allow you to work on rotation, anti-rotation, flexion and lateral flexion, anti-flexion and anti-lateral flexion, extension, anti-extension, stability, and dynamic stability ... in other words, all the parts and things your core does.
And it does way more than you might think, which is why crunches don’t cut it!