529 account for estate tax planning

Illinois has a 4M lifetime exemption and it is fairly easy to end up exceeding that in central Illinois with the price of farm ground hitting 20k/acre for flat black dirt.

In my small town there is one CPA who is revered as a tax god and he brushes off my idea as not feasible. I say the 529 can be used to get several hundred thousand out of the estate, but he says the 10% penalty is too harsh to justify using the strategy.

For the person I'm thinking of though, there are 5 beneficiaries who are designated in trust to divide the whole estate. I can't see how forfeit of 10% of any gains in the account overrides paying estate tax.

He would get tax deferred growth. He could still control the assets. Estate taxes are 10% at the 4M threshold.

Plus the heirs might not pay the 10% penalty if they use the assets for a child or grandchilds education. They could convert some of the money penalty free to Roth also.

How is this a bad idea?