China's state-owned banks may be offering the world some of its biggest and hottest initial public offerings, but investing in them can be a leap of faith.

Even analysts at rating agencies, whose job it is to determine the banks' stability and credit-worthiness, say they have only a weak grip on what's really happening inside these black boxes.

"We come out of a ratings meeting with executives who talk about reform, but then we read the paper and see stories about scandals," said David Marshall, head of Asian financial institutions for Fitch Ratings.

But as China's major banks go public, data on their operations is gradually becoming more accessible. Though it only scratches the surface, it's enough to give a sense of their potential weaknesses.

At a conference in Hong Kong on Friday, Fitch's Marshall offered an insightful reading of some numbers on China Construction Bank, which had an $8 billion IPO last October.

Shares in CCB, China's third-largest lender by assets, closed at HK$3.55 on Friday. The stock debuted at HK$2.35.

Marshall's assessment of CCB: despite its listing on the Hong Kong Stock Exchange alongside sophisticated international banks like HSBC, the CCB still basically functions to collect money from China's vast population and pump it into state-owned enterprises.

According to the bank's listing prospectus, it has more than 14,000 branches and 146 million active accounts. "Active" in this case means that an account holds more than 100 yuan, or about $12. "Inactive" accounts bring the bank's total to more than 300 million, says Marshall. CCB has total deposits about $476 billion. That means the average account size is about $1,500, but the majority are much smaller.

What's most telling, though, is how the bank distributes its funds. Its loans are as large and concentrated as its deposits are small and fragmented. The average corporate loan size is over 25 million yuan, or more than $3 million.

Nearly half of corporate loans are over 100 million yuan, or $12 million. That's far greater than at most Western banks, which make a large portion of their loans to small and medium-sized companies, according to Marshall.

"These banks haven't moved far from their old business model of shoveling money to state-owned enterprises," he said in an interview with MarketWatch. "That business model isn't sustainable in the long-term. It's not going to achieve good profit margin. Banks need a more diversified portfolio."

Another eye-opening statistic for investors: CCB has more than 300,000 employees.  which has several times CCB's assets. In a market-based economy, a company can trim staff if it runs into tough times. But in an economy that's still nominally socialist, China's state-owned companies play a welfare role.

If China's economy turns downward, investors in its state-owned firms could be funding a nationwide social safety net. And investors won't have much say in the matter, because the government usually keeps a majority stake when its firms go public.

Yet another source of concern, say analysts, is the disconnect between the banks' head offices and local branches. Their networks are so far-flung that reform-oriented big-city executives often have little knowledge of, or control over, what goes on there. It's at the local level that most corruption takes place, and where bad loans get covered up.

Finally, Marshall worries that China's listed banks haven't made enough progress in the field of risk analysis. He cites the CCB's own data showing that it has only 12 people evaluating new loans at its head office, and more generally suffers from a low ratio of risk experts to the total number of loans being made.

Many analysts suggest that investors look at less-hyped Chinese bank plays, such as China Merchants Bank. Though smaller, it's widely reputed to be better run, and controls a large chunk of the country's nascent credit card market. CMB is planning a roughly $2 billion IPO in Hong Kong in September.

As long as China's economy keeps booming, none of the banking industry's problems are likely to become critical. But when the growth spurt ends, banks could be in for some serious pain. When that happens, investors' IPO fever could turn out to have been the start of a much more serious malady.