Yeah, they shouldn't be as able to get kills as easily as a foot archer with the same bow, which currently they pretty much are.
The thing is cRPG is a team game, sure on a public server it doesn't always feel like it but certainly in more organised group vs group matches. The game is not all about k:d ratio and topping the board.
This is where support classes come in and to my mind Horse Archers should be one of the most flexible support classes, but they shouldn't be full on killers.