You may take on more risk depending on which etf you choose. An example - iShares GSCI Commodity-Indexed Trust ETF tracks an index of commodities. But it is heavily weighted in energy (nearly 70%). Therefore, your portfolio may take a bigger loss if the energy commodities were hit by unforeseen problems.
It is more diversified than investing in just one commodity such as the iShares Silver Trust Fund which only holds silver bullion.