The Federal Reserve's decision to make future tapering dependent on economic data leaves stocks and bonds vulnerable to major tumbles should that data come in stronger than expected, says David Robin, co-head of financial futures and options at Newedge brokerage.

The Fed decided Wednesday to taper its bond purchases by $10 billion a month. Future tapering won't be automatic, but will be "data dependent," Fed Chairman Ben Bernanke said.

In other words, the economy's strength, particularly the labor market, will determine the Fed's pace of tapering.

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"If we get a surprise piece of data, if all of a sudden in February we've got the unemployment rate at 6.6 percent and three or more consecutive 200,000-plus non-farm payroll [increases], I think all bets are off," Robin tells CNBC.

Such numbers could send stocks and bonds tumbling, as investors anticipate accelerated tapering, he explains. "I think that the Fed has opened the door to exaggerated reactions due to data dependency."

A short-term range of 3 to 3.25 percent for the 10-year Treasury yield appears "safe," Robin notes. But, "you can't rule out that it goes beyond 3.25 in a payroll-type reaction."

The yield stood at 2.92 percent late Thursday.

To be sure, some other experts were impressed with how the Fed handled its policy change.

"I think the tapering together with the language that the Fed used in its announcement should be encouraging to investors," John Carey, a portfolio manager at Pioneer Investment Management, tells The Wall Street Journal.

Editor’s Note: New Video Exposes a ‘Great Retirement Heist’

Related Stories:

• Morningstar's Johnson: 'Now Is a Great Time' to Taper
• Pimco's El-Erian: Taper Forces Fed to 'Walk Quite a Tightrope'