Is there any risk associated risk to holding the ETF for short term or for an extended period, apart from the risk that the underlying assets go to zero? In other words, is it more risky than owning a stock, or even the pool of stocks in the index that the ETF mirrors?
Yes. An added risk with an ETF against owning the pool of stocks is that the value of the ETF can deviate from the value of underlying pool. Now this deviation is generally tiny even in times of major stress like the 2008 crisis and there are mechanisms that should keep them close. But this is an important difference to understand especially for trading over very short terms (minutes to seconds). On the other hand, owning the full pool would generally be very expensive and time consuming for an individual investor, so it is not recommended.
Owning an ETF may be more risky than owning a particular stock under some measures. It depends on the risk of the assets in the ETF portfolio. Generally, though, the diversification from owning multiple stocks in an ETF makes them less risky. A particular stock may drop to zero unexpectedly, but a diversified ETF of many stocks is much, much less likely to do so.
Finally, some very small ETFs might add significant trading risk too as the bid/ask spreads can be rather large, but larger ETFs can be as easy to trade as the most commonly traded stocks.