The Emperor was astounded to find a world so well-ordered and prosperous [Macragge], and realised at once that Roboute Guilliman was a Primarch of unsurpassed ability and vision.
pg. 13, History of the Ultramarines, 5E Space Marines Codex
Conquered more worlds than any other Primarch, left the worlds better than the Ultramarines found them, was so tactically brilliant that it frightened Horus...the more I read Guilliman's entry, the more I wonder if the Emperor just made Horus the Warmaster as a joke to tell his Custodes while working on the Imperial webway. After all, last I checked in the 5E Rulebook, Horus was his most trusted servant...
I could go on and on about how much I dislike the work of Matt Ward, but when Roboute Guilliman divides the legions into chapters to prevent any one individual from holding so much power, yet those chapters all consider Marneus Calgar to be their "spiritual liege"...
Few Chapter Masters could contemplate refusing a request from Lord Macragge, and throughout the campaign fresh squads and companies had arrived to place themselves under Sicarius' command.
pg. 48, The Zeist Campaign, 5E Space Marine Codex
...well, I just smell a rat is all. If Guilliman meant to prevent power from being held by one individual, yet all of these other chapters willingly submit themselves to Calgar, that means Guilliman failed in the long run.
But again, I could just be speaking out of dislike for Matt Ward. I wouldn't have a problem with the Ultramarines or their various ICs if he would just stop shoving them in my face like it's the best intellectual property to be written in the history of ever.