What sort of things can go wrong?

Scaling: The nemesis of plasma physics has been scaling up something that works at small scales. The reason is that plasmas have long-range forces. They are not local.
Plasma loss rate: If the plasma is lost too quickly, we end up losing the inflation, and we revert to a magnetic sail or dipole field.
Waves: If the SW plasma can “slip by” without transferring momentum, we lose our thrust.

Scaling: The nemesis of plasma physics has been scaling up something that works at small scales. The reason is that plasmas have long-range forces. They are not local.