That's also in Modern, a much less interactive format. Karrthus seems too risky in Legacy.
Edit: Watched the video. Not impressed.
High variance hands lead to many mulligans, trying to win with 5-6 cards vs good Legacy decks. One was a mull to 3.
The manabase gets wrecked by Wasteland.
Pyre is often a turn too slow to be relevant.
Easily gets disrupted from doing the thing with brewer's advantage lost.
Doesn't necessarily win even if you do the thing and cheat out Emperion.
Losing 1-of
Realmwalker meant the deck had no potential winning lines with just Dwarven Recruiter, so it just became a 3-mana
Worldly Tutor without Grist. And the Grist + Recruiter line was never used, seemed hard to assemble.
Duergar Assailant seems much better than
Bloodfire Dwarf, except against Elves and Goblins. The 0-mana activation is a big difference and it doesn't kill your own board.
I'm really not sold on
Warping Wail and
Spatial Contortion. Even in the matchups where you'd want it, they never did enough (in this stream or others) to actually win games. It's better than having no interaction but they're worse than the equivalent colored spells every other deck is playing. I also think not bringing in
Steely Resolve more vs fair decks was a mistake, losing to removal on Magda and Valiant too often.
Overall I think the deck just lacked interaction and lost to the opponent playing interaction or resolving problems this deck can't deal with. I think the 3-color versions with more interaction seem a lot more promising.