Bank executives love to talk about how mobile is the key to their future and how customers are shifting business from branches to their phones. Recent customer surveys, however, show banks still have a lot of work to do on their mobile apps.

Thankfully for banks, most customers are not quite to the point of dumping their bank if its mobile app stinks and switching to a competitor.

In the latest review of the best and worst mobile banking apps, the consumer site MagnifyMoney.com tallied customer ratings and said the top performers were Simple, East West Bank and Capital One Financial, plus several credit unions that share a tech vendor. Customers praised their apps' ease of use and reliability.

Citigroup and UMB Financial were near the back of the pack and the banks have no one but themselves to blame, MagnifyMoney.com said.

But Citi and UMB should not panic yet, as they still have time to fix things in a rapidly shifting market, said Jim Miller, senior director of banking services at J.D. Power, which was not involved in this study.

"Customers that are dissatisfied with their mobile app, it's not immediately pushing them out the door," Miller said. "The mobile app landscape changes probably every six months. Your bank may be behind now, but six months from now, maybe they will have a leading service."

Most banks are not yet at the stage where the mobile app is a primary concern, said Nick Clements, a former Barclays executive and co-founder of MagnifyMoney.com.

"It's not an immediate thought that the earth is burning if your app doesn't work," Clements said. "But if you have functionality that's constantly breaking, you're at risk. At a minimum you need to stay even with everyone else."

The main reason banks should improve their mobile apps is because members of the millennial generation will rely on them, Clements said.

"Millennials are incredibly important for the future of banking," he said.

For its survey, MagnifyMoney.com combed customer ratings submitted on the iTunes and Google Play sites for the week of Nov. 20. It then created a composite 1-to-5 rating system (with 5 being the top score) based on various factors like ease of use and reliability. MagnifyMoney.com looked at ratings for the 50 largest banks and the 50 largest credit unions, plus a handful of online-direct banks.

Citi and UMB fared poorly. Many Citi customers complained that the app crashed when they tried to make a deposit, and it limits customers' mobile deposits to $1,000 per day. Andrew Brent, a spokesman for the $1.4 trillion-asset Citi, said, "we know there is always room for improvement."

"Client adoption of our apps continues to grow significantly," Brent said.

The $16 billion-asset UMB, in Kansas City, Mo., hasn't updated its app since 2011 and it does not offer mobile deposit, Clements said.

"I went and looked at UMB's app, and it was miserable," Clements said. "It looks like it's 10 years old and doesn't have the functionality of other apps."

UMB is in the process of developing a new app with an expected release date in 2015, President and Chief Executive Mike Hagedorn said. The new app will include mobile deposit.

"We are excited about the enhancements, convenience and security it will provide our customers," Hagedorn said.

Consumers who reviewed Capital One's app said they liked its "Sureswipe" password feature (which uses a connect-the-dots page in lieu of an alphanumeric password) and praised the app's reliability. Several credit unions that use Digital Insight's app also ranked well in the customer comments.

It is still difficult, if not impossible, to get a clear sense of how mobile apps are affecting banks' bottom lines. Institutions are not required to break out how they are spending or earning on mobile banking, and they do not. But they are throwing out broad figures that give a general sense that mobile is booming.

Beth Mooney, chairman and chief executive at the $87 billion-asset KeyCorp, for example, said at the Goldman Sachs U.S. Financial Services Conference this week: "We've … seen strong adoption of our mobile offering in mobile-deposit transactions, which has doubled from the level that they were just one year ago."

John Stumpf, chairman and CEO at $1.5 trillion-asset Wells Fargo, said at the Goldman conference that the bank has between 13 million and 14 million mobile customers, and is the San Francisco bank's "fastest-growing channel of distribution."

And Brian Moynihan, CEO of $1.5 trillion-asset Bank of America, said at the Goldman conference that "in the mobile banking area...; we've developed up to 16 million-plus customers. That's 2 million more than when I was here last year, and we added 30,000-plus last week."

But none of those comments are sufficient to form an accurate picture of whether a bank's mobile strategy is working, Clements said.

"I haven't seen a lot of disclosure yet," Clements said. "There are still a lot of banks that won't even tell you the number of checking accounts they have opened."

As more customers flee branches for routine transactions, whether a bank's mobile app works will become more important to executives. Until then, many bankers will continue to view mobile as an experiment that's not yet worthy of top-priority status, Clements said.

"Apps are very different from getting your next checking-account product or credit card out," Clements said. "People are becoming more demanding of apps and the risk of them leaving is greater if you don't stay up to date."