Well, there's nothing about Guilliman not being able to make mistakes or having flaws, he just hasn't screwed up yet. We know he's made plenty of powerful enemies in his reshuffling of the High Lords, due to his flaw of not exactly being the best people person. Sure, he's great at organising, but he's not exactly the best at convincing people of things. He tends to just sweep people up as he goes, and leaves plenty of people broken in his wake, or left behind. His method is "right, so this is how things are going to go, for I am Primarch Guilliman, and therefore smart". He doesn't exactly win over his detractors, he just steamrolls over them, and either replaces them, like the High Lords, or swoops in to save them with his reformations (Dorn, the Primaris Marines, etc).
He might be on a roll now, because he's still riding high on being a mythological figure returned to life, but sooner or later something is going to turn for him. He's going to push the Mechanicus too far, assuming they'll go along because he's Guilliman and therefore right, or something will happen with the Primaris leading to rejections occurring, or his political enemies will strike. After all, this is 40k, and as the saying goes "if you think this ends well, you haven't been paying attention".