I've been playing Circle for a year and I've noticed a general trend when trying to use models outside of the conventional range of net-lists: many of them simply ask "May I, Please," instead of taking a role on the table. To an extent the heavy goats all suffer from this problem, and the units with heart-eater certainly do ask well. The trend towards Wurmwood or Una2 and the former trends of Kruegar2 and Bradigus indicate that players like the process of taking action and will favor lists and warlocks that allow them to take active roles in the battle. Grayle and Moshar especially seem to ask permission of their opponents.
I am aware of the general thinking about "questions" versus "answers" list-building, but that binary doesn't actually reflect how games work in play. Instead, I think the game needs to be understood better in terms of active versus passive list choices. For example, a Juggernaut is an active piece - it does things - where as the Gnarlhorn asks to do things - counter-slam - but its associated stats don't allow it to be active outside of that question and so the piece is essentially passive. Their similarities in point cost seems possible in the abstract, but disparate in reality. Sentry Stones do active things, playing a role in every engagement, and so they find a space into lists. Most of Circle's mid-range beasts and our infantry (esp. Druids and Tharn) are passive.
Heart-eater is a fine example of this problem, and I believe its nature as a passive ability explain why Tharn have little traction. The ability's effects look superior to Weapon Master, but in reality, Weapon Master is an active ability, driving play and performing tactics. The Death Wolves ask permission to use their abilities of the enemy (can you bring infantry?), and when permission is not given become a passive unit, while the Black Frost Shard with active support abilities, active protection ability (Stealth), and Weapon Master, is an example of a similar unit that takes an active role in the game.
As a Celtic mythology fan, I'm disappointed in the way the Tharn play, mostly because their evocative abilities don't play out (and they don't have chariots). Kromac1's Cuchulain warp spasm is evocative, but it is not a defensive response anymore, and heart-eater dies on the table almost every time...
So what would you do to enliven the Tharn? Lower Points? Turn the Lord of the Feast into added support solo augmenting heart-taker? A cool theme list to empower the ability?