Income investors won't get mouthwatering dividend yields from buying most exchange-traded funds (ETFs). For example, the highly followed SPDR S&P 500 ETF Trust currently yields a little over 1.5%. It would take a boatload of money to fund retirement at that level.

However, there are other alternatives that you might not have heard about that can generate significant income. Here are three ETFs that offer ultra-high yields of 10% or more. 

1. iShares Mortgage Real Estate ETF 

The iShares Mortgage Real Estate ETF (REM 1.23%) sports a 30-day Securities and Exchange Commission (SEC) yield of nearly 11.5%. This yield, by the way, reflects all interest earned net of expenses during the most recent 30-day period. It provides a great way to compare ETFs.

As its name indicates, the iShares Mortgage Real Estate ETF focuses on real estate. It primarily owns U.S.-based real estate investment trusts (REITs) that hold residential and commercial mortgages.

REM's portfolio currently includes 32 stocks. Its biggest holding by far right now is Annaly Capital Management REIT, which makes up more than 16% of the ETF's total portfolio. Other top positions include Starwood Property Trust REIT, AGNC Investment REIT, Rithm Capital, and Blackstone Mortgage Trust REIT.

2. VanEck BDC Income ETF

The VanEck BDC Income ETF (BIZD 0.90%) stands out as another ultra-high-yield ETF with which many investors might not be familiar. Its 30-day SEC yield is nearly 10.9%. The fund has also delivered an impressive total return so far in 2023 of 17.5%, which puts it well ahead of the S&P 500. 

The "BDC" in the ETF's name stands for business development companies (BDC). The VanEck BDC Income ETF tries to replicate the performance of the MVIS U.S. Business Development Companies Index, which tracks a basket of 26 BDC stocks.

Ares Capital, the largest publicly traded BDC, makes up more than 20% of BIZD's net assets. Other top holdings include FS KKR Capital, Blue Owl Capital, Hercules Capital, and Golub Capital BDC.

3. iShares Emerging Markets Dividend ETF

Another iShares ETF could also be intriguing to income investors. The iShares Emerging Markets Dividend ETF (DVYE 1.39%) offers a 30-day SEC yield of nearly 10.5%. 

As you might have suspected, this ETF focuses on exposure to companies in emerging markets. It seeks to track the Dow Jones Emerging Markets Select Dividend Index, which includes 100 established companies in emerging markets with high-dividend yields.

DVYE's holdings are more equally weighted than REM and BIZD. Its top position is currency management-services company REC Limited. Other major holdings include Brazilian oil and gas company Petroleo Brasileiro, Taiwan-based electronics maker ASUStek Computer, Brazilian utility Companhia de Saneamento de Minas Gerais, and Taiwan semiconductor maker Sitronix Technology.

Are these ultra-high-yield ETFs buys?

Investors who are focused solely on income could find all three of these ETFs attractive based on their current yields. However, there are a few important things to keep in mind.

First, the income generated by these ETFs could fluctuate. For example, the dividends paid by the iShares Mortgage Real Estate ETF have declined in recent years. 

Second, there's a risk that any of these ETFs could lose money on an overall basis. As a case in point, the total return of the iShares Emerging Markets Dividend ETF over the last 10 years was negative 16%.

Third, even if the ETFs generate a positive total return, you could still make more money by investing in lower-risk funds. None of the ultra-high-yield ETFs beat the total return of the S&P 500 over the last five-year and 10-year periods.

Sometimes chasing a high yield can work to your disadvantage. I don't view any of these ETFs as great picks. My view is that income investors can get better total returns in exchange for somewhat lower yields.