I'm surprised they actually moved forward with this idea. From the time this index was announced and tracked on March 6th until the sales began the GDAX index has fallen by ~50% in a bit less than a few months.
Volatility increases tracking error; this benefits traders but hurts investors.
Furthermore, AUM is not delta neutral. When an asset goes up its ETF attracts capital; when it goes down the ETF tends to lose capital. Since ETF economics are a measure on assets under management (AUM), ETF sponsors generally want their ETF to go up.
Even the prospectus says "The long term expected value of your ETNs is zero": http://www.ipathetn.com/US/16/en/details.app?instrumentId=25...