An interesting card in Edge that was completely decimated by the release of Scarlet Keys.
The initial choice is whether you want to lower deck consistency to increase deck consistency. I'm not going to pretend to have run the numbers to see whether it's worth it for that, but Rogues' specialisation in Exceptional cards adds some amount of efficiency for finding those.
But then, rather than supporting the 'highlander' archetype a little more, Scarlet Keys came very close to destroying any reason to run this card at all.
A consistency tool like the similarly-named Underworld Market comes close to blowing it out of the water. It isn't hard to stack 8-10 weapons in that deck and focus on economy or niche cards in a fighter. It's a little harder to do so with investigation tools or evasion tools, but not impossible. And you can mix and match any of the three. The problem is that this is very hard to reconcile with Underworld Support, which would make your Market deck far, far less consistent.
Then we have the other Rogue consistency tool, Friends in Low Places. As a customisable, running a single copy makes it considerably less cost-efficient, costing 10 to fully upgrade one copy rather than two. On top of that the card itself benefits from running multiples of the cards you want to catch with it. As it only digs six (or nine) cards, if you can run two copies of what you're searching for, you're much more likely to hit it. Even in the best case of using it to dig through a smaller, Underworld Support deck, for exceptional items, you've still made it much less efficient to upgrade.
There's also the other two inefficient upgrades, Damning Testimony and Honed Instinct. While Damning Testimony isn't especially bad with Support beyond the inefficiency, using Support completely locks you out of what's probably Honed Instinct's best upgrade, letting you run it as a pseudo-myriad card.
On top of that, there isn't a single card in Scarlet Keys that makes you look twice at Underworld Support positively, except, perhaps, the limit-one Dirty Fighting, or wanting to quickly cycle Clean Sneak and not waste a whole 8xp on two copies. The only exceptional in the set is the Market itself. Even the high-cost ally is non-unique, so there's little reason not to buy two of him if you want him. Something like Embezzled Treasure is much better if you can manage to stack two copies.
It's especially a shame because the other permanents from this cycle got at least a small boost. Down the Rabbit Hole is great with customisables. Salvage is a pretty considerable boost to Short Supply decks. Guardians at least got a handful of interesting items which you can consider with Geared Up, and Vincent is an excellent new user of it regardless. Though there's little for Forced Learning decks, you can at least make the case that an extra investigate tool or a way of searching for your key cards can add some consistency to such a large deck.
I really would like to see a card or two boosting the 'highlander' archetype, perhaps even explicitly, in the next set. So here's hoping.