I looked at the Iowa 529 plan, doesn't seem bad. It has various index funds with a 0.26% expense ratio, which isn't amazing, but it's only about 0.1% higher than the lowest-cost plans out there (such as California and Vanguard/Nevada). The state tax deduction will probably pay for the difference, but do the numbers to be sure.
I don't know why people would tell you to invest in a taxable account against a 529 as the investment grows tax-free.
The reasoning here would be that we don't know what the future will hold. Maybe your kid will earn a huge scholarship and won't need the money. Maybe they decide they want to learn a trade instead of going to college. Maybe they join the Army and get the GI bill to pay for their education. Maybe a populist revolt causes state governments to give more funding to colleges so tuition won't be as high as you're expecting.
With a taxable account you pay the same relatively low capital gains tax on the earnings whether you use the money for college or not. With a 529 account, you pay no tax on the gains if the money is used for college, but you pay tax at regular income rates (plus 10% in many cases) on the gains if the money is not used for college.
Take the likelihood of not needing the money for college, multiply it by (your expected retirement tax bracket + 10%) and compare that to your expected retirement capital gains bracket. If the first percentage is lower, go with a 529. Else go with taxable. We did start a 529 for our son, but every family is different.
Do max out the retirement accounts first. For one thing, IRAs have a favorable provision where you don't pay early withdrawal taxes on money taken out for education, so you do get some tax benefits if you need that money for school. For another thing, you can always make your kid take out loans for education if necessary; not so for your own retirement.