Some places (eg most states in Australia) it's not legal to carry fuel in those rear carriers due to increased risk in a rear-end accident. They fit a standard jerry can, but they're only intended for water.

For an off road trip, where putting them on the roof could increase the risk of a roll-over but the chances of being rear-ended are minimal, I'd be inclined to ignore that law. For a "flee the hurricane" type scenario on a crowded highway, I'd put them on the roof.

The safest option would be to fit a long-range or additional fuel tank, but cost and availability is a factor.