I tried the T Rowe Price one you mentioned because although I've used other calculators in the past, I'd never tried a Monte Carlo one for college. (I have for retirement.)
I was shocked at how much more money it said I needed to put in each month compared to other calculators. Poking around I found it assumes a 4.90% return on the market. That's definitely quite conservative, and it's for the actual stock market, not some kind of return for an automatic mix of stocks and bonds as you get closer to matriculation. Of course as the time for matriculation gets closer, average market returns mean less. The difference between 7% and 4.9% over a year is not that significant. But if you're planning for a baby's future fund, I personally find this assumption too conservative over 18 years. It's nice to oversave, but not if one is sincerely struggling in the meantime.