A remarkable number of ETF's have underperformed the underlying commodity it purports to track
The marketing literature of most ETFs explain that the ETF will track the underlying commodity less

Seasonal factors
Supply/ demand on the underlying commodity
Trading commissions
Fund manager charges

Eoin Treacy's view Commodity Index tracking funds are not the same thing as Equity Index tracking funds because the underlying asset is structurally different. A buy and hold strategy with an equity is relatively straightforward because the share does not have to be stored, will not decay (other than through dilution) and has reference to a business which produces cashflows. Commodities need to be stored, many are perishable and they have no cashflows. Commodities trade on futures markets which adds time to the valuation equation.

This means that an Index which attempts to track the performance of a commodity, particularly one which is subject to highly seasonal supply/demand swings will have great difficulty approximating the futures price. This is the reason why most such funds track an index which factors in costs rather than the spot price. Gold tracker funds are an exception, benefitting from the relative size and depth of the market as well as gold's immutability and ready access to storage facilities.