In short: The more treasure it returns to the top of the deck, the more Mandarin slows you down.
Think of it like this:
If you return your Copper to the deck and purchase a Mandarin, you increase the number of turns between buying the Mandarin/other good $5 card and playing it. Meanwhile, if your opponent also opens 5/2 but gets a good $5 card (let's say Witch) he can potentially play it on turn 3. This is probably bad for you, since he's reaping the benefit of playing the Witch earlier. It slows down your momentum, pretty much.
That said, there are some decks that will like having a Mandarin and don't mind getting started slower, but those are probably pretty rare. Usually, the on buy effect of Mandarin is better late in the game when you want another shot at buying Province with your Gold or something.
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Actually, winning the split in a plain witch game with no engine depends much more on luck than on 1 turn, because you have to reshuffle your (most probably two) witches twice before the curses are gone. Since later on, a reshuffle takes about 2-3 turns, this means, you would roughly deal about 0.5 curses less. Still, the mandarin helps you on turn 4-6 to get one more witch before your second shuffle, so you would rather win one turn and thus lead by 0.5 curses. So I claim, you could actually more often than not win the curse split AND have a nice card for late game (more reliable than witches if there are no villages).
If there are villages on the board, the whole thing changes, since you can maybe play your witches more often than every other turn. Thus, it's more about who's first to play 5-6 witches, and less about who can do it more reliably.
Yet, I would probably not go for Mandarin if there is an IGG, since it deals curses faster than witches, i.e. every time you buy one. So this card doesn't depend on reshuffles and thus you are more likely to lose the split if you "waste" one turn.
Of course, strong terminal attacks like ghost ship stringly seem to favor not opening mandarin, whereas nonterminal draw cards like lab or stables seem to favor mandarin.
