Van Agtmael said there are fundamental concerns. China's economic growth, which had been about 8 percent to 10 percent on average for the last 20 years, is now above 11 percent, an unsustainable level, he said.

Inflationary pressures are being seen, he said, noting that consumer prices rose more than 6 percent in August and September. "Inflation will continue to be higher than people expect so that the Central Bank will at some point be forced to act a little more forcefully than they have this far when they've acted rather feebly," he said.

The so-called "through train" plan was announced by the State Administration of Foreign Exchange (SAFE) in August, enabling individual mainlanders to invest in Hong Kong shares. That plan has been delayed. It's part of the reason the H share market rose so much, said van Agtmael, but its impact is likely to be disappointing.

Some expected that Chinese retail investors "could now invest more or less in an unlimited fashion in Hong Kong," and expected that that would present an arbitrage opportunity, he said. "I don't believe that ever was the intention; the intention is to take a little pressure of the A share market in a controlled way."

For a number of reasons, this bubble will pop reasonably soon, he said. With the rest of the world slowing, China is going to face a situation where the external environment isn't good, he said.

Landesman of ING said that while a US or global slowdown will have less of an impact on China's economy now than it would have in the last cycle, it will still have an impact.

"If there's a global economic slowdown, it's not good for anybody," he said.