On the surface, fixed-index annuities are attractive. You won’t lose your initial investment, and you will receive a guaranteed minimum rate of interest and have a chance of collecting slightly more if the stock market does well. But that’s where things get murky.

The investment is typically tied to an index — often the S&P 500, but, sometimes, a proprietary index you’ve probably never heard of. Your annual return is often capped at, say, 2.5 to 4 percent. That cap can change from year to year, and the insurance company may use other levers to limit your upside. And the performance of the investment may trail the underlying index, because it often doesn’t include dividends.

Then, there are the less than obvious costs. The email pitch sent to Mr. Phelan proclaimed that fixed-index annuities “often cost nothing at all!” That’s not true either, annuity experts said. The costs are baked into the overall returns. There are a rising number of fixed-index annuities available through registered investment advisers, which some experts said offer a better value. But if the adviser is charging 1 percent or more for their services, “it might erase any benefit,” Mr. Dauenhauer added.

Digesting all of the details is challenging. “One of the things you don’t want to do is buy them when you are emotional,” said David Lau, founder and chief executive officer of DPL Financial Partners, which helps financial advisers find insurance products for their clients. “Now is not the time to rush out and buy an annuity because you are feeling panicked.”

Other fairly complex annuities are being offered, too. One is called a buffer annuity, which allows investors to capture some gains, while curbing some, but not all, losses. Then there are variable annuities, which may promise guaranteed income. They are essentially a portfolio of investments tied to an insurance policy, which can be expensive, yet they are frequently being offered to teachers and other public school employees.

Mr. Lau suggested focusing not on what is being pitched, but on the problem you’re wanting to solve. Maybe that’s a guaranteed stream of income, or ensuring you have enough to cover your fixed expenses.

Stan Haithcock, an agent who calls himself “Stan The Annuity Man,” said he’s been selling plain vanilla annuities that offer guaranteed income, but even these come in different varieties. One is a single-premium immediate annuity, which pays a guaranteed lifetime stream of income in exchange for a lump sum of cash. The other is a deferred income annuity, which starts the income stream at a future date, not immediately.