The apparent downside is the two life you have to pay, and it being a dead card when you draw it in the late game
One particularly damning thing to note is this chart: This table shows that with 7 Swamp and 7 Plains, you will have the required Plains to cast a turn-3 Territorial Hammerskull only 85.9% of the time (on the play, in absence of mulligans or card selection)
And as one of Magic's best infect commanders, Skithiryx, the Blight Dragon easily threatens to kill any player with one or two hits, provided that you have your poison/proliferate engine going, or a well-timed Hatred
Full recap table at the bottom
Brief Strategy Description Create pilots to crew big vehicles Decklist 100 cards Commander
though the archetype has many powerful enablers like Sleep and Opposition, Hylda is essentially the only payoff for the archetype