But IB is on a growth curve, with customer assets up 244 percent over the past five years, when the S&P 500 was up about 60 percent.

The firm’s cut-rate pricing of one dollar or less per trade is attracting advisors, of course, but Sanders says the ability to trade in multiple global markets and currencies is also a big draw.

“If you want to trade around the world, we are one of the only solutions,” he said.

That assumes advisors can figure some things out on their own. Advisors say the platform has so much capability that it can be daunting to learn. But they do give IB kudos for improving ease of use in recent years.

“There’s a huge learning curve at IB,” said Wes Gray, chief executive of Alpha Architect in Broomall, Penn., which has about $500 million in client funds at Interactive. “It’s not exactly made for your grandma.”

Gray says he doesn’t need the “bells and whistles” offered by other custodians, and for someone like him who needs good trading technology, transparency, and the ability to access and manage data, “IB dominates.”

Like Gray, Evans used Interactive personally before starting his RIA in 2010. About half of his $180 million in assets are at Interactive, with the rest at Schwab and TD Ameritrade.

“All [those firms] are very good,” Evans said, but cost-wise IB is generally better for accounts larger than $100,000—the level where IB waives its $10 per month inactivity fee. Smaller accounts must generate $10 in commissions, or be charged the difference.

“And if you borrow on margin, it’s always better to be on IB,” Evans said, because of the firm’s low lending rates.

“One reason for [Interactive Brokers’] success is that they’re not trying to compete with the major custodians,” said Christopher Winn, managing principal at AdvisorAssist LLC, an advisor consulting firm.