I don't see anything unstable there. It's surely very long, and the wing is way behind the centre of mass, so the centre of aerodynamic forces will be behind it too. Cannards look to be put way ahead of centre of mass too.

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.

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

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