Flat extension cords are fine — if they're the right gauge.

Not inherently, but it depends on the cord’s rating, not its shape.

Flat cords are usually designed for light-duty use: lamps, phone chargers, small electronics. They often have thinner wire (16 or 18 gauge) and a lower amperage rating. Round cords tend to be heavier-duty (14 or 12 gauge) and can handle more current.

The real safety risk is using a cord that’s too thin for what you plug into it. A flat cord running a space heater will overheat. A round cord running a lamp won’t.

Flat cords also get walked on more, and the insulation can wear out faster. But if you’re using it in a low-traffic area for low-power stuff, it’s fine. The shape is a red herring.

Buy the right gauge for the job, not the right shape.