Every once in awhile, even savers cruising on autopilot need to check their engines.

Retirement investors poured another $69 billion, a record, into target-date mutual funds in 2015, according to data firm Morningstar, bringing the category to a total of $763 billion in assets. Did someone say juggernaut?

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Automatic contributions into workplace savings plans account for a big chunk of target-date funds’ growth. And by definition, the funds’ mandate is to provide a portfolio that adjusts its risk level over time, so investors don’t have to bother picking investments and rebalancing their mix of stocks and bonds. Compared with excessive trading and other investing mistakes people make, most experts say, the funds offer novice investors professional management at a relatively low cost.

Looking at investor returns, which simulate how actual investors fared in the fund using cash flows, the Morningstar team found that investors on average slightly outperformed the funds’ total 10-year return. All good there.

Some experts are warning about some turbulence on the radar, however.

The funds vary widely in how they adjust risk over time, and that could be a big problem, says Michael Kitces, director of wealth management for Pinnacle Advisory Group in Columbia, Maryland.

When investors are more than 25 years from the target retirement date of the funds, virtually all of the funds allocate a high percentage of assets to stocks. However, at the target date stated by the fund — usually reckoned to be when an investor is likely to retire — there’s a wide gap, with funds ranging from about 60 percent stocks to less than 25 percent.

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Kitces says he worries about funds that hit the target date with a high percentage of stocks — closer to the 60 percent territory — and then dial down stock exposure to very low levels by the end of retirement. The reason: Retirees drawing income off their portfolios compound any early losses and a declining percentage of stocks means little chance the nest egg can recover over time.

The speed of the stock reduction matters, too. Funds with a mandate to reduce stock exposure relatively quickly could take severe hits if those years coincide with bad years for stocks, Morningstar researchers found.

Finally, says Morningstar’s Jeff Holt, the funds haven’t yet tackled the mechanics of drawing income in retirement.

“The big question mark that target date managers are still trying to figure out is retirement income,” he said.

For now, you’re on your own to decide when, and how much, to withdraw at retirement.