Wow, that's a wide range of rates! I guess a lot depends on the why, when, & how the chicks are being helped out. Some are having difficulty getting out because of their own inherent issues, and those probably won't have enough of the Right Stuff to live for long anyway. But others may truly be strong enough to survive, but other circumstances are just preventing them from breaking out of that shell. When those are helped out, they do better.
I don't do any electric incubating at home, my only experience with them is at our County Fair. For 17 days I volunteer in our Poultry Tent, and there we have an incubator & brooder, and have set eggs to hatch every day of the Fair. So I've seen a lot of chicks hatching there. We have one volunteer who is very experienced in helping chicks hatch, I'm sure she has a great success rate if I tracked those chicks she helps hatch.
A few years ago I brought home some of those chicks, including one tiny one that had to be helped out of its shell. One of my sons took a great liking to it, named it "Flea". But poor lil' Flea never grew like the others, and died a few weeks later, causing my son a lot of grief. He's known & loved other chicks & chickens of ours that have died, but Flea's death really affected him strongly, poor kid.
And now I'm raising several batches of chicks that local teachers have hatched in their classrooms from fertile eggs I supply. There was one "helped" chick in this group, and it was like Flea, not growing & then dying. That's what made me curious to start this thread.
I can see the wisdom in not helping when you have large batches of eggs to hatch. But with smaller clutches I figure it's best to first provide the optimal conditions, and then when there's a chick who will die anyway without help, go ahead and give it a chance. But tell your kids not to get too attached for a while...