Black Market's unique, killer cards are not all that unique from distinct, killer reshuffle luck in terms of having fate hand you a winner.
Sure they are. If you open 5/2 you clearly have an advantage on a lot of boards, but there are a lot of places where that advantage might be surmounted. For example, you open 5/2, but your Witch misses the first shuffle and I play my T3 Witch before you on T5. More importantly, there are often a lot of tactical adaptations I can try (by cycling instead of +actions, get a second trasher, etc.) in order to attempt to optimize my build against your luck.
With the admittedly uncommon killer unique Black market cards, this sort of equalization may not be possible at all. For an obvious case, say I pull Possession out of the Black market on a board with Masq or Amb. How exactly do you tactically adapt to that? Sure I might take an extra turn to get you a Masq/Amb and take a few Possession plays to line up with whatever are the key cards (Colonies, Goons, villages, etc.), but you pretty much cannot change things. Likewise, if there is no trashing in game except for one Chapel, there just is no way on a lot of boards to compete against a thin reliable deck.
Getting there first is important, often game deciding near equal skill for a lot of other things ... but very rarely do things swing so heavily on just ONE shuffle (stuff like Possession, Kc/Bridge, etc. might come close, but those are rare).
Why does it matter if it's one reshuffle or two? I accept this point: Black Market into Chapel as the only trasher on a board with Conspirator, Worker's Village, and Lab is 100% win rate, opposite player's decisions no longer matter, all in one fishing out of the Black Market deck.
5/2 Embassy into Gold into Gold into Gold into Province into Province into Province into Province into Province also induces a 100% winrate where the other player's decisions no longer matter. It requires more reshuffles to be controlled, for sure, the characteristics of each reshuffle are critical for the games where Embassy is ridiculous to actually be ridiculous. The the end result that one player got a 0% winrate irrespective of his own decisions is the same. No engine has time to try to ramp up try for the Duchy split on those mornings when Embassy wakes up and decides "Today is a good day to mimic Stash"
The only difference is perception, the false decisions that the player facing Embassy perceives as being relevant. You think, maybe the Gold or Embassy will miss the reshuffle
next time, so let me think about what I'm buying, let me maximize my chances of winning. But you don't know the entire time that none of it will matter.
With the turn 3 Chapel or turn 3 Witch, you might know that none of it matters right away, although only because you can look through the Black Market deck and find out your fate is actually sealed. But does playing out the game offer any less value to you than playing out a loss against the Embassy player? You can still play pretend, you can still think about scenarios in which what you buy does matter, and decide what you would buy in those scenarios. You will enjoy the same choices as the Embassy victim, and end up with the same loss as the Embassy victim. Maybe that's what you're into. But you can also do something the Embassy victim can't do, you can just resign, right away, start a new game.
I'm not going to address Possession Ambassador handing over Province because that's a little bit what if the blue moon rises on the 29th day of February and I get KC-Masquerade-Goons'ed by Elvis, who is still alive, was just set aside with a Haven that was trashed by a Procession of Prince, in terms of total frequency.
