UNCONSTRAINED bond funds can own debt instruments of any type — long- or short-term, high- or low-quality, corporate or government or whatever, in any country and currency — to try to enhance returns. With interest rates near record lows and investors eager to bolster yields, enthusiasm for these portfolios, also known as nontraditional funds, seems boundless too.

The level of assets in nontraditional bond funds tracked by Morningstar rose about 80 percent last year, to $123 billion. They continued to attract fresh money early this year, reaching $132 billion in assets at the end of February.

How richly have shareholders been rewarded for these funds’ ability to go anywhere in search of the best performance? Not very. The average nontraditional bond fund returned 0.3 percent last year and 1.1 percent in the first quarter of 2014.

Unconstrained can mean unprofitable, it turns out, and it can mean riskier too, versus conventional bond funds, investment advisers warn. While the freedom to hold any type of bond can make unconstrained funds valuable portfolio additions, it’s essential to use the freedom wisely, they say. Some managers may be taking on too much risk to satisfy investors who are fed up with the low income of many bonds.

“People tell us they’re not looking at regular bonds; they want unconstrained. They want 5 to 6 percent yields,” said Laird Landmann, co-director of fixed income at the TCW fund management company. “It’s an end-of-cycle dynamic. When people are reaching for yield, it’s a warning sign that they want to take more risk at the wrong time. I’m not sure that unconstrained funds will protect investors if the cycle turns.”

Running unconstrained funds is tricky: They are effective only if managers are imaginative in allocating assets and are neither too bold nor too timid.

“Some funds might have wanted to benefit from the idea of being unconstrained, especially in 2013, when the average bond fund lost money,” said Todd Rosenbluth, director of fund research at S&P Capital IQ, part of Standard & Poor’s. “In theory, these funds can go anywhere, but whether they do,” and where they go, needs to be examined case by case, he said.

Aaron Izenstark, manager of the Iron Strategic Income fund, whose performance has ranked in the top 10 percent of unconstrained bond funds during the last five years, sees many of these funds heading in one of two general directions. They either load up on higher-quality instruments like Treasury bonds. he said, which adds interest-rate risk if the maturities are long, or they concentrate on lower-quality debt and add credit risk.

“The average unconstrained fund is a simple bond fund,” he said. “It gives the manager leeway to do what he wants, but there’s not a lot of creativity in most of them.” For a fund to be truly unconstrained, in his view, it would have to be able to do more than just go into any market. It would also have to be able sell bonds short to bet on falling prices and use derivative instruments to hedge exposure to different risks, like rising interest rates or falling currencies.

Where Mr. Izenstark finds too much creativity is among debt issuers who he contends are dumping paper of dubious quality, including securitized subprime mortgages. the scourge of the 2007-9 financial crisis, on credulous investors.

“You have to be very careful in some of these spaces,” he warns. “People want it, so you have someone make it, even if it’s absolute garbage. These things that are coming out, it’s not that it’s not a great time to be in them. It’s a great time to sell them.”

Mr. Izenstark had almost no allocation to securitized loans or to government bonds at the end of February, the most recent period for which such information was available. The portfolio was 87 percent invested in corporate bonds, compared with 12 percent for the average nontraditional bond fund that Morningstar tracks.

Mr. Landmann, who is also a co-manager of Metropolitan West Strategic Income. a TCW fund with the best five-year results among unconstrained bond funds, noted that when funds in the group have produced especially high returns in recent years, it has been by taking greater credit risk. That adds another type of risk that shareholders may not realize. Price movements in stocks and lower-quality bonds are highly correlated — both are manifestations of risk-seeking — so some funds may not provide the diversification for a broad portfolio that conventional bond funds do.

“Unconstrained funds are supposed to have strategies that provide high returns and protect you from trouble,” he said, “but 50 percent of the universe is just credit funds or equity funds in another guise.”

The unconstrained fund of which he is a co-manager has lower exposure to high-yield debt than other funds in the category, Mr. Landmann said.

The ability of managers to own anything can make it hard for investors who are still interested in this niche to choose a suitable fund.

“Everything unconstrained goes in the same box,” Mr. Rosenbluth said, “but unconstrained, by definition, does not go in a box.”

That makes it vital to inspect the contents before buying.

“Try to understand where they’re investing,” he said. “If you combine it with other bond funds, you need to know how it fits in. Are its holdings long- or short-term, government or credit, U.S. or foreign? Understand where they’ve gone most recently and how quick the turnover is.”

Because these funds can go anywhere, Mark E. Balasa, chief investment officer of Balasa Dinverno & Foltz. a financial advice firm in Itasca, Ill. encourages investors to focus on those run by firms best equipped for the journey.

“One of the keys to using an unconstrained fund is to understand the organization behind it,” he said. “You need one that can pull the trigger on all those pieces — high yield, emerging markets, bank debt.”

He pointed out, though, that such grand scope has drawbacks. “You need size for economies of scale and research capability,” he said, “but you need to be small enough not to move markets.”

Mr. Rosenbluth advises investors to keep an eye on the operating costs of unconstrained funds. The average one has annual total expenses of 1.7 percent, very high for a bond portfolio. Three funds that S.&P. recommends for being run well and economically are MainStay Unconstrained Bond. Putnam Diversified Income Trust and the one from BlackRock.

The BlackRock and Putnam funds have annual expenses of less than 1 percent on at least one share class with a minimum investment of $10,000 or less, and a retail share class of the MainStay fund has annual expenses of 1.3 percent. The corresponding figures are 1.4 percent for the J.P. Morgan fund, 1.3 percent for the Pimco fund, 1.2 percent for the Iron fund and 1 percent for the Metropolitan West fund.

Mr. Landmann warns that this isn’t a time for bond funds or their shareholders to take on undue risk. “The time to be aggressive in a nontraditional fund was a year or two ago,” he said. “Now it’s time to be conservative. The traditional way to protect yourself from a turn in the credit cycle is long-duration Treasury bonds. That’s still true; it’s probably the best hedge.”

A version of this article appears in print on April 6, 2014, on page BU12 of the New York edition with the headline: When Bond Funds Think Outside the Box. Order Reprints | Today’s Paper | Subscribe