Bitcoin's reliability remains suspect despite its growing legitimacy as an investing option, a currency expert said Tuesday.

The New York Stock Exchange launched a bitcoin index on Tuesday. It marks the first exchange-calculated index for the currency, with a real-time price tied to the Coinbase marketplace.

Despite bitcoin's increasing use as an asset—including an exchange-traded fund launched earlier this year—its reliability makes it tough to classify as a real currency, said Win Thin, senior vice president and global head of emerging markets at Brown Brothers Harriman.

"It's hardly a reliable store of value," said Thin in a CNBC "Closing Bell" interview, referencing bitcoin's price fluctuations.