Two reasons: leverage costs will drag the performance of the 3x to worse than 3x over time, and in big drops there's a chance that the 3x ETF may actually implode like the XIV ETN did a while back. Those work in your favor if you're short but may or may not be worth the cost of borrowing for your situation.

> in big drops there's a chance that the 3x ETF may actually implode

Huh, I didn't consider this possibility because I can't imagine the underlying index (this one's related to the S&P 500) going up enough for that to happen. But then again, I suppose those speculating in XIV didn't expect those volatility spikes either.

For me, though, I figure the downside is capped for the relatively small amount of money I put into this 3x inverse ETF: if the fund goes all the way to $0, then I've only lost the original investment. (But also, if it goes to $0, that's probably a good thing for the rest of my portfolio.)