The key effects in dominion are not typically money, you can almost always get money into a deck without too much trouble.
For a standard big money deck, getting bigger and better treasures is usually a winning difference.
Yeah, but there is a reason the Big Money tends to only be competitive on sparse boards with few options. In any event, on a Big Money board, you want something more powerful than Terminal Silver-ish thing.
Mandarin is subtly different than -1 card - it's one card on top of your deck. This seems trivially different, but it can be an advantage in certain situations, and these situations really define Mandarin's utility. After all, it would be extremely unfair to say Courtyard is only as good as Moat, correct? I go into this more below. (I think it's clear you understand this, but it's not the same as -1 card and I'm annoyingly pedantic sometimes)
Yes I do get this, Chris, but the difference with Courtyard is that it is $2 and so is Moat. For $2 buys you +2 cards -1 action and some bonus for Courtyard that is something your money or higher odds of hitting your engine. For Moat that bonus is protection from attacks. For an early game Mandarin, you are looking at a terminal just barely better than Silver that actively decreases your odds of hitting engine.
Now sure, later in the game this analysis does not hold. Using Manadarin to send back cards is big, but we are talking about as an opener. At most you will start with Mandarin, some $5, and a $2 (barring Baker or Nomad camp). It really is giving your self a terminal silver (on average) that slows down your cycling.
There is one situation you're leaving out - you can top deck another terminal action. Combine this with the Mandarin opening guaranteeing another $5 buy on the next turn, and you've got pretty strong buying power early on without all of the drawback of a terminal Gold. If you want a deck with two other terminals early on (e.g. Mountebank, Witch), strong buying power, but without the risk of collision or the necessary $6, Mandarin is a cool way to do this. It isn't exactly game breaking, but it's occasionally useful and should be kept in mind.
With Witch or Mountebank, you just open straight to those. If have no other card, I can't collide period. Getting a curse or better curse/copper into my opponent's deck a shuffle earlier is far more important than having a gimped gold. Think about it this way. We both open 2/5. You go Mandarin -> Witch, I go Witch. I'm going to give you your first curse a turn sooner on average. So if you go first I can curse you on T3 or T4 before you shuffle (T3 you buy a witch and shuffle, after T4 you have at least 5 cards left in deck). If I go first, you cannot curse me before I shuffle again (after T3 I have 0 - 2 cards left in deck, you can first draw your witch on T4). I'm pretty likely to end up winning the curse split 6:4 and very unlikely to lose it 4:6.
Essentially, delaying the play of the curse giver is pretty close to taking an extra curse. Buying embargoed gold is worse than buying silver for money density.
It absolutely does, because right after you bought Mandarin you bought another card, presumably an Action, for $5 that you can top-deck with it. The effect has immediate utility, and in some ways is best in the early game since you're really getting Mandarin for the Gold-like buying power a shuffle (or two) earlier than you otherwise would.
Yeah, but that delays the useful $5 yet another turn. The curse givers are obvious, delay playing them and the other guy has better odds of winning the curse split. That is big without curse trashing. Major trashers, like Count or Trading post, also don't want to wait and potentially miss a shuffle.
You need to have a really good reason for wanting to hit $6 to delay playing your other $5 buy.
Mandarin is a decent card, I actually like it and think it is a bit underrated, largely because it is so bad as an opener. Gaining it off University is pretty good on a lot of boards, using it as virtual coin while setting up a 7 card hand to start next turn is also good. But opening it? No, I like seeing my power cards sooner & more often too much for that.