I don't feel like 529s are that great, honestly.
Most states have weak tax breaks for them. You incur a 10% penalty on them if you use them for anything other than education expenses (plus taxes).
Capital gains aren't terrible -- and any Mustachian is going to have tons of space under current tax code for free capital gains, unless they are retiring "late."
But otherwise, the nominal tax benefit is not worth the disadvantages that tying money into an account takes. I'd much rather pay some small cost in order to have taxable investments, so when the time for college comes, I can either cash flow it (and then use the investments for ER) or otherwise have flexibility. A 529 decreases that flexibility.
If your kids are very young, you are also assuming that college costs 15-20 years from now will be the same as they are now. I'm less than convinced they will be. You also are assuming that your kid will go to college. And that if they do, they will have to pay (and not get scholarships).
However, if you're maxing retirement accounts (which offer much better tax benefits than 529s do) and also able to have taxable accounts and have extra money, a 529 seems like an ok place to park money, I guess. But I don't really understand the hype. They seem nominally better than taxable investments, assuming all assumptions come through as expected a decade or more into the future for most people.