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They're fairly unstable.

The trouble is that at the wingtips, if the wing twists up, aerodynamics forces tend to push it more up. So there's a positive feedback loop.

Another problem is that if you yaw slightly, the yawed backward wing gets more straight on airflow, and the forward wing gets less, causing a stronger yaw force. Again, positive feedback loop.

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> Another problem is that if you yaw slightly, the yawed backward wing gets more straight on airflow, and the forward wing gets less, causing a stronger yaw force

According to geometry, this problem must also occur with conventional backward swept wings.

The geometry makes forward swept wing destabilize further, while classic design self-stabilizes (or oscillates).

That said, slight forward swept had been used for years in many planes, it's the more extreme angles that get the fun mentions.

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