It’s ranged minions in general. Between Widowmaker, Heartseeker, Ki Beholder, the neutral ranged minions you pick up and, of course, your BBS which can create a Heartseeker every 2 turns, there generally aren’t enough tools is most opponents’ decks to deal with so many ranged minions.
Here’s an example. Let’s say you are P1 against Vetruvian and you drop a Vale Hunter (or you’re P2 and drop Widowmaker) on the back column. Their best answer is Siphon Energy (or maybe a flyer like Putrid Dreadflayer/Wings of Paradise) to contest it.
Then you Killing Edge it and chew up the next 3-4 minions they play while they try to get close enough to dispel it using a minion or kill it with tiger.
A buffed ranged minion on the back line is the Achilles heel of Vetruvian because there are so few answers. To make it worse, sometimes you aren’t even offered a siphon energy - or, if you managed to get one or two, you might not draw them.
Other factions tend to have more tools to deal with ranged but, again, sometimes they don’t have them, and by the time they draw it, you will be so ahead on board and tempo that they’ve already lost.
Then there are combinations like Bloodborn Spell (create a Heartseeker) + Inner Focus + Killing Edge + kill one of their minions. You’re spending 2 cards to remove one of theirs and leave a massive threat on the board that simply must be answered.
Even if they kill it - that’s 2 of your cards for 2 of their cards - a fair trade, and this is the “best” possible case for them.
There’s just not enough ranged hate readily available in Gauntlet and Songhai can run away with games as a result.