But some advisers challenge this point of view, saying it is almost akin to market timing. “You could look at any asset class at any point in time and position it in a way and understand why it’s outperformed or underperformed,” said Scott Stackman, managing director of private wealth at UBS Wealth Management.

Here is Mr. Rasmussen’s argument for caution in three areas:

A model past its prime

During the financial crisis, Mr. Rasmussen worked at Bain Capital, a leading name in private equity. One of his jobs was to collect data on deals by Bain and its competitors to determine why some had done well and others had not.

The more profitable deals were the least expensive ones, he found. The cheapest 25 percent of deals accounted for 60 percent of the funds’ profits. The top 50 percent accounted for just 7 percent of profits. The difference was the price paid for the company. This was not solely for the obvious reason that paying less is better, but because private equity funds typically borrow 60 percent of the purchase price, which affects a company’s profitability.

Mr. Rasmussen said he admired the success Bain had in the 1980s and ’90s, but began to question whether the private equity model it had helped pioneer was still sustainable.

When early private equity firms bought relatively small companies at a discount and loaded them up with debt, the amount of leverage on the company was still about four times the company’s earnings before interest, taxes, depreciation and amortization, a measure of profitability known as Ebitda.