Banshees and Avatons have extremely slow movement speed, presumably to balance out their special abilities.

However, they are slow to the point of making them non-threats. Simple movement even with Slowness is enough to run circles around them, let alone normal walking or sprinting. A vanilla skeleton has far more threat than either of these new undead types alone.

To fit with these entities not having "feet" but rather floating tatters and ectoplasm, the Banshee and Avaton should float around like the Ghost Ray or Vex, descending into an appropriate range when using their melee attacks. They would still be relatively slow, but this increased maneuverability would greatly increase their identity as floating, ghostly monsters and even allow them to pose a threat to players with flight or skybases.

Ideally, since you have already designed the movement system for the Ghost Ray, giving these monsters that kind of movement wouldn't be too difficult (compared to remaking them from scratch).