Games like Assassin's Creed Unity, The Crew, and Far Cry 4 no longer have store entries on Steam. Right now, nobody seems to know if this is being done intentionally by Ubisoft or if it's an issue just with Steam. A look at SteamDB shows a rather interesting back and forth with these games throughout the day.
Earlier, when the games disappeared from Steam in the UK, PCGamesN was told by Ubisoft that people can purchase the games from Uplay instead.
“We’ve been in discussions with Valve about Assassin’s Creed Unity but for the time being the game is not available via Steam in the UK,” Ubisoft told PCGamesN.
“In the meantime, UK customers wishing to purchase the game digitally can do so by visiting the Uplay store, our retail partners or other digital distributors.”
“In the meantime, UK customers wishing to purchase the game digitally can do so by visiting the Uplay store, our retail partners or other digital distributors.”
If this is an issue where Ubisoft is pulling their games from Steam in order to push Uplay, I can't imagine any scenario where that actually works out in their benefit. Then again, poor sales of the PC versions of their games could be used in the future to just not make a PC version at all. So, who knows! Again, it's all just speculation at this point on what is really going on.
There's no way to prove if Steam is more effective than Origin or Uplay for selling those publishers games, i'd guess that more copies of Dead Space games have sold on Steam than on Origin however, and likewise Black Flag probably sold more on it than on Uplay.
Prices for those AAA games are pretty high though, and i'd imagine they sell fairly well on game key websites because of it.