I've hatched a lot. Perhaps at least 200 this year. In my experience, if all the conditions are right and they don't explode out on day 21, there is very little chance of saving them.
Even if you do an assisted hatch, it just isn't worth it. You might get a breathing chick out but it usually can't stand up and is eventually lost.
I don't assist often but I used to if I thought it was my fault (poor incubation conditions) I would try to assist and I probably only saved 3% of them. And I don't really want to base my breeding program on less than vigorous chicks.