Warpstone Flux Rating: ⭐️⭐️⭐️⭐️
4.5/5 stars, rounded down. The rules are very good.
His accomplishments before, during, and after the Heresy are a tough pack to pick between. Here Guilliman is presented as the leader of Ultramar, a statesman, a warrior, a leader, a wise man and intellectual, and master strategist. He is all of these things on the battlefield too.
His stat line comes with the handy WS=7 for sibling battles. Re-rolling to hit rolls of 1's thanks to Calculating Swordsman on the second round of combat onward gives him yet another edge here. Sure, he is not Horus. He isn't even Russ or Ferrus. But he is certainly highly capable among his siblings.
The armour is good enough, and the re-roll of the first invulnerable save is powerful. His weapons in melee are great - choosing between S+1 or S=10 with different special rules to help amazing, and the brutal x2 for the Hand of Dominion is going to be powerful in all situations coupled with AP=1. The arbiter is very handy as a side arm, but with AP=3, its in the territory of being okay without being amazing.
His warlord trait grants an army wide boost to Ld, as well as selection of a phase in which to play an additional reaction. Both are powerful boosts to the army. His preternatural strategy takes what Alpharius and his sons should have and makes it Ultramarines territory (yes, I'm bitter). Fleet, counter attack, furious charge, and stubborn are all there to choose between, just not on consecutive turns.
Tanks and tar pits like Vulkan might prove an issue. Horus (and Ferrus, and Russ) still whips him into submission quickly enough. But he is powerful among his brothers, and brings amazing boosts to the army. There's little not to like here.
Categorically one of the more powerful primarchs. His army boost in combination with the rite of war makes for some game defining moments.
Difference to First Edition.
His weapons from first edition are here, but the calculating swordsman aspect is not what it was. He's different, for sure, but here in second edition, I regard him as excellent overall.