If the plane is not yaw stable, then as it slows down it will yaw one way or the other. That yaw will cause the spin break to be unreliable and instead of just the nose dropping straight ahead, a wing will drop with the nose. Especially in upright spins. Inverted spins will probably be cleaner as the fin is not blanked as much.
My original Episode even with the fences I have on it is that way. On the new one I made the fences slightly larger. Now it is very reliable on spin entry.