Because the issuer simply promises the return, an ETN can track hard-to-hold exposures such as certain commodity or volatility indices with no tracking error from holding costs. There is no portfolio, so there is nothing to sample or roll imperfectly.
The trade-off is total issuer-credit-risk. If the bank fails, the ETN holder is an unsecured creditor; holders of notes issued by Lehman Brothers discovered this in 2008. There is no ring-fenced collateral behind the note.
Two further risks are specific to the wrapper. Issuers can suspend new creations, after which the note can trade at a large and persistent premium untethered from its index, and many notes contain acceleration clauses letting the issuer redeem early at a value set on a single day.
Related: issuer-credit-risk, etf, counterparty-risk, structured-product, leveraged-etf, creation-unit