Which:
1) He ended up bargaining with a Warp Entity (Tzeentch), masquerading as a friendly Warp Entity, to get that far, otherwise he wouldn't have pulled it off.
2) Basically ruined what remained of the Emperor's post-Horus Betrayal plan by wrecking all the technology around the Golden Throne.
3) Distracted the Emperor, who now has to defend against a Warp portal in his throne room.
4) His message was totally unnecessary in the first place. If the Emperor didn't already know by some method, dozens of others brought that knowledge later. The best you might say is that Magnus' warning led to a shit load of loyalist legionnaires and Ferrus Manus to die on Istavaan IV a little bit earlier that they might have.
4) Killed tens of thousands from the psychic backwash, and blinded the Imperium's Astropaths right before the Ruinstorm was unleashed. (I think, the chronology isn't all that clear between the invasion of Calth and what's going on in the rest of the Imperium at that time.)
5) Pissed off Disappointed daddy something fierce.
6) Created the believable context for Horus to deceive Leman Russ which ultimately got his homeworld wrecked and the vast majority of his Legion slaughtered and imprisoned him in the Eye of Terror in a shape formless, feckless and insane.
7) His lasting impact on the Heresy post-banishment is visiting various primarchs/key note characters as one of his fragment forms giving them advice, or chastising them, or complicating the information they're getting. IIRC one of the primarchs, I think Lorgar, even calls him out on it during one his visitations. "YOU'RE giving ME advice?"
If you compare "desired outcomes" to "actual outcomes", Magnus was a total failure in that. I mean, if he had brazenly stormed the Throne Room with psychic powers, I'd give it to him! Bro had a plan, and he enacted that plan under his own steam. But that wasn't his plan. Everything Magnus tried to do just dug a deeper hole for him, and he didn't do anything exceptionally proactive. And that's sad sack. The most gifted and subtle psyker and sorcerer short of the Emperor reduced to the equivalent of banging on the Emperor's front door with a sledge hammer until it fell down.
I mean, I get it. They wanted to show how Magnus' arrogance and hubris cost him, his Legion, the Emperor, Terra and maybe the whole Imperium dearly. If Magnus could have been trusted he might have foiled 50%+ of Lorgar and Horus' warp sorcery and turned the tide of the Heresy truly in the Imperium's favor. But....Mangus never did anything cool or well as it bears on the rest of the universe. Yeah he did some dank sorcery in the obligatory intro invasion that makes up the first 80 pages of most HH books. But then he went right to sucking, and worst, sucking in the lamest ways possible: getting duped by stuff he should have known better about; taking the brute force approach rather than the subtle approach and gravest of all: sitting in his pyramid in Prospero having a good sulk while his world and his legionnaires die all around him....AND THEN changing his mind at the last minute, only to get his spine snapped like a twig over Leman Russ' knee. Oi vey. I hate on a lot of the non-loyalists primarchs, but at least you can say someone like Konrad Kruze did something with his time that actually demonstrated his mastery over his art. You can basically sum up 90% of Magnus' activities as "projecting his rockin' spirit body everywhere."
They want you to sympathize with Magnus' tragedy, but all I feel is disappointment and no small amount of contempt, since he created most of the problems that were his undoing.
I think Magnus would have been written better as a power-obsessed evil sorcerer primarch, if my choice was between that and the wishy washy progressive wizard with a hubris problem that we got.