If you’ve opened the Trump Accounts app, the pitch for investing is hard to resist.

Enter a $250-a-year contribution, and the app shows the user would have $19,000 by age 18 or a whopping $878,000 by age 55. Bump it up to the $5,000 annual max, and the numbers jump to $271,000 and $13 million, respectively.

That eye-popping figure comes straight from the government’s own projection on TrumpAccounts.gov, but it rests on an assumption of the S&P 500’s historical annual return of more than 10%, sustained without interruption for 55 years. While that 10% has historically been the case, Morningstar provided CNBC with data showing U.S. stock market returns could be lower over the next decade, closer to an average return of 6.3% per year.

That being said, financial planners want parents to see the full picture before they start dreaming of what feels close to a trust fund, or at the very least, a nice nest egg, for their kids.

Trump Accounts, the tax-advantaged investment accounts for children created under President Donald Trump’s tax law that officially launched July 4, have drawn attention for a headline promise: that a child could retire a millionaire off contributions their family barely notices. The accounts function like a traditional IRA, but during the “growth period” that runs from birth through the year before a child turns 18, special rules apply.

Eligible babies born between 2025 and 2028 receive a one-time $1,000 seed deposit from the U.S. Treasury, and families, friends, and others can collectively add up to $5,000 per year in after-tax dollars, a limit indexed for inflation after 2027.

So what could a family actually build? And what should they understand before treating any app projection as a full financial plan? Here’s how four financial experts break it down—including Adam Vega, a certified financial planner and managing partner at Avance Private Wealth Management, who is weighing the accounts for his own newborn.

“We’re going through these nuances together,” he told Fortune.

How much could a Trump Account really be worth?

The four advisors who spoke with Fortune landed in a very similar range, and they got there using a more conservative return assumption than the Trump administration.

Pam Krueger, a registered investment advisor and founder of the advisor-matching platform Wealthramp, ran the numbers for a family that maxes out their accounts. Add the $1,000 government seed to $5,000 a year from birth through age 18, and the family has contributed roughly $91,000.

Assuming a 7% long-term annual return—her benchmark for money invested in the stock market over a lifetime—”that account could grow to roughly $185,000 by age 18,” Krueger told Fortune. Left untouched after that, with no further contributions, “it could grow to more than $1 million by age 45.”

“But that child could have much more by the time he/she is in their mid 40’s,” she added. “Time in the market is doing the heavy lifting. That’s the power of compounding growth.”

Mitch Hamer, founder and lead advisor at Intersecting Wealth, modeled the same idea for his own 5-year-old son, whose account he recently funded. Also using a 7% return, he projects those maxed annual deposits reach $1 million at age 45 and $3 million at 60. At 8%, which he also considers defensible, the figures climb to $1.4 million at 45 and $4.5 million at 60. Those totals, he notes, would be built on just $200,000 of contributions by age 45.

“A long time horizon and no interruption of compounding is a powerful concept in personal wealth accumulation,” Hamer told Fortune.

And that’s the lesson financial experts emphasized over and over: The contributions are almost beside the point.

“Here’s the part that should stop people in their tracks,” Matthew Chancey, a certified financial planner, tax strategist, and founder of Tax Alpha Companies, told Fortune. Of the $1.5 to $2 million he projects a maxed account could reach by age 55 at a 7% return, only about $91,000 came from the family.

“The other $1.5 million or so came from time,” he said. “That’s not a rounding difference, it’s the whole story. Which means the only real question isn’t how much you put in, it’s whether the kid can leave the money alone long enough for time to do what time does.”

Krueger made the same point using percentages: Using a 7% assumption, more than 90% of the account’s eventual value comes from decades of compounding, not deposits.

“The real engine isn’t the deposits—it’s time,” she said. “That’s why starting early puts someone so far ahead.”

The caveats parents keep underestimating

While all of that sounds great, it’s nearly impossible to know or completely accurately project exactly what will happen in the stock market over the coming decades.

“These are not guarantees,” Krueger said. “Even a small one- or two-percentage-point difference in long-term returns can change the outcome by hundreds of thousands of dollars in either direction.”

Then there’s tax treatment, which trips up families who confuse “tax-deferred” with “tax-free.” Unlike a Roth IRA, withdrawals from a Trump Account are taxed as ordinary income, and the account converts to a traditional IRA the day a child turns 18. That means withdrawals before age 59½ can trigger a 10% penalty unless an exception like education or a first-home purchase applies.

“Many people hear ‘tax-deferred’ and mistake that for ‘tax-free,'” Krueger said. “They are not the same.” Still, she added, “I wouldn’t let the tax tail wag the dog.”

But one of the major risks all the financial experts marked isn’t the market or the IRS. It’s what happens at 18, when the child gains full control of the account.

“The day they turn 18, you go from being in charge of that account to being on the sidelines,” Chancey said. “Legally, practically, and completely.” Every family swears they’d never touch it, he said, until a hard year in the kid’s 20s arrives.

“Next thing you know, 40 years of tax-free growth quietly goes to solve one temporary problem,” he said.

Hamer’s takeaway is that education has to start early.

“Education on the money and what it stands for, what it took to earn, is just as important as the compounding itself,” he said.

Vega agreed the control question is the account’s biggest limitation: “Most people are not too financially responsible when they turn 18.”

Where does a Trump account fit alongside a 401(k) and a 529?

Many parents also wonder whether Trump Accounts then replace a traditional retirement or college savings account. But financial planners said it’s additive and doesn’t replace either of those savings mechanisms.

But advisors did say to maximize an employer 401(k) match first.

“If your employer matches your contributions, that match is free money,” Chancey said. “Fund a Trump Account for your kid before you’ve captured every dollar of your own 401(k) match, and you’ve made an expensive mistake dressed up as good parenting.”

From there, the planners generally sequenced the Trump Account and a 529 next. Krueger would prioritize a 529 if college is a likely goal, for its education tax benefits, followed by a Trump Account.

“Its biggest advantage is that contributions don’t require the child to have earned income,” she said. “That helps. I’d want to put the dollars where they’ll work the hardest.”

Meanwhile, a growing number of employers are sweetening the pot. Companies including Uber, Intel, IBM, and Nvidia have pledged to contribute to workers’ Trump Accounts as a benefit. Many employers will add up to $2,500 a year per employee, which counts toward the $5,000 limit.

“We know free money is the best kind of money,” Vega said.

When a Trump Account actually wins

The Trump Account’s edge, the financial experts agreed, is flexibility and timing.

It beats a 529 for families who aren’t sure whether their kid will go to college, because a 529 penalizes non-education withdrawals. It beats a custodial Roth for young children, because a Roth requires earned income and most kids don’t have a paycheck, but a Trump Account can start compounding at birth.

Where a Trump Account doesn’t win is once a teenager starts earning real money. A custodial Roth’s tax-free growth generally beats the Trump Account’s tax-deferred, ordinary-income treatment over a multi-decade horizon. And a 529 still wins for clearly college-bound families in states offering a meaningful tax deduction.

Financial experts suggested converting a Trump Account into a Roth IRA in early adulthood, when the young account owner’s income and tax rate are low. Vega has modeled it for clients. Even a $2,500 annual contribution for 18 years, he said, “then purposely converted to a Roth is valued upwards of $2 million at the child’s retirement age,” and tax-free.

The bottom line, as Chancey framed it: The account is a great tool, but it can’t be treated as a holistic financial plan.

“The plan is whether the kid can actually leave the money alone for five decades, so it can do what it’s built to do,” he said. “That’s not something the tax code decides. That’s something the kid decides, potentially one hard season at a time.”