The first thing that should come to mind is "if it seems too good to be true....".
Yes, there are risks of holding too long, and more often than not the price will be declining due to
contango. Just take a look at the
prospectus, where it says in bold that you are likely to lose some or all of your investment if you hold long enough.
Many people mistakenly believe it's perfectly inversely correlated to the stock market, and many times that is the case, but all you have to do is look at yesterday to see that it isn't always true- TVIX was down almost 7% despite the market also being negative.
This isn't meant to be a recommendation either way, it can certainly have spurts where it can be a home run, but do your homework.