Designed to deploy when triggered by a side impact, the curtains may not activate in a slower rollover event. Some vehicles do include rollover sensors that deploy the curtains when the vehicle tips, but others do not, and this information may not be available to car buyers.
NHTSA wants side-curtain airbags to help prevent people from being ejected from the vehicle in a rollover crash, a problem which leads to more than 5,000 deaths a year. Rollover crashes in general cause one-third of passenger vehicle fatalities. Read about the NHTSA's research into rollover crash accidents here.
There are problems, however. Rollover events are less predictable and relatively slow compared to impacts, and curtain airbags may not stay inflated long enough to provide protection through a multiple rollover crash.