Past performance, we are repeatedly and rightfully reminded, is no guarantee of comparable performance in the future.

One mutual fund sector for which it is likely to be especially hard to sustain and duplicate past performance is United States real estate. Over the last 15 years, real estate funds have returned 10.5 percent annualized, which is second in that period only to stock funds that invested in India-based securities, according to David Kathman, a Morningstar analyst. After this long, upward march, real estate funds still have pockets of strength but face several lurking risks.

Most of these funds invest primarily in publicly traded real estate investment trusts, or REITs. Exempt from most taxes, REITs must pass on at least 90 percent of their net income to investors each year. REITs generate income from shopping malls, industrial warehouses, multifamily apartment buildings and residential home mortgages. They might even hold real estate used for corporate data centers or for self-storage facilities.

REITs tend to be big borrowers, said Mr. Kathman, and policies of the Federal Reserve Board have “helped all debtors.” Many of them have used low rates to refinance and reduce debt, which had risen in the years leading up to the financial crisis of 2007 and 2008. As measured by the ratio of debt to total enterprise value, “most REITs were in the low 40s,” noted David Wharmby, a portfolio manager of the Oppenheimer Real Estate Fund. Today, the ratio has declined by about one-fourth. “The persistence of low rates for so long has been an advantage,” he said.

Compared with the microscopic returns on bank savings rates, REITs, with an average trailing 12-month yield of 1.85 percent, have been an attractive yield play.

But after 15 years as a market favorite, what is the value equation for REITs now? Paul Adornato, an analyst with BMO Capital Markets, said one way to answer that question was to compare the REIT market prices with their net asset values — the value of their underlying properties. Currently, Mr. Adornato said, REITs sell at a 7 percent premium to net asset value, or NAV. Historically, they’ve traded anywhere from a 15 percent discount to a 20 percent premium, he said. By that measure, the current pricing is reasonable.

“Based on NAV, we would not say that REITs are currently expensive or cheap,” Mr. Adornato said.

Other measures look worse, however. Joel Beam, a senior portfolio manager of the Salient Tactical Real Estate Fund, cited the ratio of share price to REIT cash flow, using adjusted funds from operations. Historically, this multiple has averaged around 16, but it has now soared to 21. His advice to individuals mulling REIT investments: “I would say if you’re going to choose a stock on your own, be careful.”

Mr. Beam saw some REIT share prices as overextended. “The REIT market is over all not cheap,” he said. The Salient fund has been adding negative bets on REITs — shorting the securities. “We’re upwards of 24 percent short,” he said, declining to identify specific positions.

Having benefited for so long from Fed policies, REITs are hostage to fears of a further Fed interest rate increase. Yet the Fed has held off from raising rates for months, and may maintain or even lower rates to stave off further market problems after the shock of the British vote to leave the European Union.

A far greater threat to REITs — and to many asset classes — is the possibility of a recession that would pinch business growth and consumer spending. But unless that happens, several REIT sectors may be appealing.

Shopping malls, for example, are often viewed as imperiled by the rise of online commerce, but some of them may be worth a second look. “The well-located Class A malls are still growing,” Mr. Wharmby said. Such “fortress” malls are typically located in densely populated high-income urban and suburban markets. He cited two in the New York area — Queens Center in Elmhurst and Roosevelt Field in Garden City, on Long Island. Queens Center is owned by Macerich, an Oppenheimer Real Estate Fund holding. Roosevelt Field is owned by Simon Property Group, the largest holding in that fund.

Such malls are rarely built today, making existing properties more valuable, said Joseph Fisher, a portfolio manager of the Deutsche Real Estate Securities Fund. Another sector with constraints on supply is industrial real estate. Warehouses and distribution centers have benefited from the expanded shipping needs of e-commerce sites.

Driven by a hunger for data storage arising from social media, mobile computing and the relentless push to digitize personal health records, data centers have been a trendy REIT subsector. But as numerous government agencies, universities, retailers — and recently, the Democratic Party — have learned, no form of electronic data storage is entirely safe from hackers. Privacy and security concerns could impede the growth of physical data centers one day, though that hasn’t happened yet. “It is a fear,” Mr. Wharmby said. So far, though, he said, it hasn’t affected “the rush to cloud computing.”

Investors who don’t own a home and want exposure to the residential real estate market can choose from a wide range of REITs that invest in mortgages. Or they can opt for a mortgage REIT exchange-traded fund such as the iShares Mortgage Real Estate Capped E.T.F. or VanEck Vectors Mortgage REIT Income E.T.F.

On the other hand, homeowners who don’t want to increase their investment in residential real estate might consider REITs focused on nonresidential property like malls, office buildings or health care facilities. Major E.T.F.s that invest in such REITs include Vanguard REIT E.T.F. and iShares U.S. Real Estate E.T.F.

REITs can provide diversification and income, but there are risks, especially in the current market. Keep your eyes open. You may not want to count on the returns of the last 15 years.