1. They overlook interchange fees. HSAs are largely transactional accounts that come with debit cards, Christenson explained. Credit unions with less than $10 billion in assets are exempt from the Durbin cap on debit interchange fees, but bigger credit unions get relief too, he said.

"HSAs even for over $10 billion are actually carved out of the Durbin Amendment, so even the larger players aren't subject to that same level fee. There's no cap on it for an HSA," he said.

2. They don't think about building a pipeline. Credit unions can tap into a new member base by partnering with a local insurance company or independent agent who sells high-deductible health plans and having them offer HSA accounts to those customers.

"You can do it on a one-by-one basis or go to the employers in your area," Christenson added.

3. They assume HSAs are tiny accounts. That's wrong, Christenson said. One objective of an HSA is to help pay health insurance deductibles, which means many people are accumulating thousands of dollars. And once former owners of use-it-or-lose-it flexible spending plans learn they won't forfeit their HSA money at the end of the year, he said, many quickly max out their annual contributions. The average HSA balance at a credit union was $2,030 in 2014, compared to $1,756 for the entire HSA market, Devenir reported.

4. They don't see the connection to millennials. "More and more employers are going to the high-deductible plans, but a lot of the younger generation is out on the private or public exchanges buying their health care," he said. "But who's educating them on the HSA component? That could be you."

5. They're ignoring their advisory arm. Members with HSAs can easily become advisory clients. After they reach a certain balance in their HSAs, a portion of their accounts typically become investable in mutual funds or other instruments, and members may need prompting and guidance, Christenson noted. The average investment account holder has a $14,654 average total balance, Devenir reported.

6. They're afraid of regulation. Credit unions are no strangers to regulation, but an aversion to that could backfire. Without training, employees may even shy away from helping members asking about HSAs.

"There really is just a fear of the transaction itself and doing it correctly," Christenson said.

Part of the problem is a looming Department of Labor rule that could make anybody paid for giving individualized advice about retirement investment decisions a fiduciary. HSAs would be subject to the proposed rule, meaning that advisors would have to sign written contracts with members and disclose commissions or other fees the advisor is getting in return for recommending certain funds or other investments for an HSA.

"A lot of folks probably right now don't even have it on their radar that they could be subject to this from an advisory standpoint," he warned.

But that's not a good enough reason to ignore HSAs, Christenson warned.

"HSAs will hold an estimated $30 billion in assets by the end of this year," he said. "It's no longer just this little side conversation."