We would say they are as safe as one can get without an explicity guarantee. First, ETF assets are segregated from fund management companies. A fund manager failure can delay funds but is not likely to result in losses (other than market losses during such delay). In 2008, there were some issues with commercial paper due to the asset-backed market collapse, and some money market funds in the US did experience losses. But the scenario in Canada was better. A bank failure would likely be the worst-case scenario. The short term nature of these ETFs helps here, but if there was a 'run' on a bank then these ETF products could see massive selling and possible losses. More likely in such a case they might be halted while the government steps in to provide support to the troubled bank if the ETF had high exposure to the problem at hand,
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