Others are unequivocal about the U.S.'s developed market status: "It's certainly not sensible to talk about the United States as a country that has profound financial vulnerability," said Ramin Toloui, co-head of emerging markets portfolio management at Pimco, which has nearly $2 trillion under management.
"Empirically, historically, there's a difference between the ability of developed countries to carry debt and emerging market countries," he said, citing factors including per-capita income and the perceived strength of institutions, as well as the dollar's reserve-currency status.
(Read more: Funds' shift away from US isn't just about the debt ceiling)
"There is a tremendous demand for holding U.S. assets," Toloui said, but added, "that is not a reason for political leaders to be complacent about economic growth or about the long-term trajectory of the debt."
Other factors also support the developed market status.
"From a soft asset perspective, it's still the best educated and most productive workforce on the planet. It's very hard to bet against that. The government has been doing its best to cause doubt about that," Sullivan said.
But he noted, from a "hard asset" perspective, the U.S. needs massive infrastructure investment.
"What's undeniably favorable for the U.S. is the level of living standards and the per capita income. After all, these things are universally very good. On that score, it's hard to compare the U.S. with an emerging economy," said Thomsen.
(Read more: Obama: Shutdown slowed economic growth, damaged US credibility)
"What's more worrying is the rate of change" on structural reforms, she said. "It's not as positive," she added.
"The countries that will win are those that undertake the most structural reform, or have the most favorable momentum of the rate of change. That momentum is much greater in Japan, China and Europe than it is in the U.S. That's what's worrying for the U.S.," she said.
— By CNBC's Leslie Shaffer. Follow her on Twitter: