What's wrong w/the Bird Defense?

From Wikipedia:

"Bird's Defence (ECO C61), 3...Nd4, is an uncommon variation sometimes played in the hope of surprising White into making a mistake in unfamiliar positions. With careful play White is thought to be able to gain a small advantage.

This defence was published in 1843 in Paul Rudolf von Bilguer's Handbuch des Schachspiels and explored by Henry Bird in the 1850s. Henry Bird himself played it as Black no fewer than sixteen times with a score of +6=2−8. Bird's Defence was later used a few times in tournament play by Siegbert Tarrasch, Boris Spassky, and Alexander Khalifman. Although it is still played occasionally as a surprise, no strong master since Bird has adopted it regularly."

What is the problem for black - White doesn't seem to have a large development gain over Black.