Horse archers act really weird for me. They accept the first command I give, like coming back to hold position or follow me, but then they go back to charging the enemy again and don't listen to any further commands.
Also, horse archery as a playing style got kind of nerfed. Like others have noted, it takes more arrows to kill people now. I don't know what those sea raiders put on their leathers but it can take up to 3 arrows to kill them, and I've got 200+ archery skill and a power draw of 3.