For me, the primary risk in commodity ETFs and ETNs are in the ones that buy futures contracts and roll them. Every time they do the fund loses money when prices are in a normal contango position. So over time, the funds will always underperform the underlying commodity, sometimes very substantially.
Bought from Cohen Mint twice in the past, when they had a presence on eBay. Which I don't think they do anymore. I didn't have any problems with my orders. The kilo copper bars I bought as a novelty arrived with no problems fairly quickly. Granted this was probably before more people knew about the place so they probably could keep up with the orders more often (supply and demand). Just speculating on that last part.