The techs to recruit these monsters is rather expensive, and King of the Wastes is even more expensive. These units should be late-game in terms of toughness, or come in higher numbers but less frequently.
Knights of Asok require alliances, they should be mega-strong units, as that's an endgame tech. Getting an ogre or troll lair activated should be as much of a gamechanger as say broadswords or pikes.
One suggestion: Yithril shouldn't need these techs to recruit monsters, getting the techs should buff monster stats for Yithril, they are the monster faction after all. This would also make them more interesting.