This is not three rules bolted together. Each covers a hole the others leave.
Armoured Trailblazers supplies the delivery and the early board. Sagitaurs with Scouts 6 inches carry Hearthkyn Warriors or Hernkyn Yaegirs 6 inches further up before the first turn. That gets bodies onto midboard objectives on turn one, which is where the army rule's Yield Point income comes from.
Farseekers supplies the information. Scornful Analysis for 1 Command Point hands [IGNORES COVER] against one spotted target to every Leagues of Votann unit in your army. In an army with an Einhyr block and a Hekaton Land Fortress, that single spend is worth far more than the 1 Detachment Point the detachment cost.
Hearthguard Covenant supplies the killing. The Einhyr block arrives via Materialisation Matrices more than 6 inches from the enemy, which is inside the detachment rule's 9 inch wound re-roll band, and Fury of the Hearth adds Strength and, for 1 Yield Point, [SUSTAINED HITS 1].
The Yield Point economy is the thing to watch. None of these three detachments generates Yield Points. Your income is entirely the army rule's objective-based schedule, which pays 1 to 4 a turn and flips your army into the defensive Fortify Takeover stance at 7. Only one card in this whole shape spends Yield Points, Fury of the Hearth at 1, so this combination reaches the 7 Yield Point threshold faster than most Votann armies and holds the defensive stance longer. If you want your army tough in the back half of a game, that is a real argument for this shape.
The obvious weakness. Nothing here is broad. Every one of the three rules names specific datasheets, so a list that does not own Sagitaurs, Hernkyn and Einhyr is buying rules it cannot use. Check your collection against all three before you commit.