In a Big Money deck, the "trash this for a Treasure" bonus will never trigger, which leaves Tragic Hero as just a worse Margrave. So TH sucks in Big Money.
In an engine deck, TH actively anti-synergizes with engines because it replaces a powerful engine component (itself) with a Treasure card that, sure, acts as part of the payload, but chances are, you bought a TH because you need drawing over money, so it removes something that you need from your deck while giving you a card that you don't need in return. So TH sucks in Engines.
It's no Scout-tier, but I would say TH is easily F-tier. I would've placed it dead last in Nocturne in a heartbeat.
P.S.: My second point for TH only applies w/o Platinums + Colonies. If you're playing with Platinums + Colonies, then TH is B or A tier.
Big Money is fine keeping the smithy +buy, if for some reason big money is actually relevant. And it costs $5, so that's something to buy if you don't have $6. And it really depends on the board if there is no way to activate it. Maybe there is Flag Bearer or something.
You say TH sucks in engines because you lose the ability to draw cards with it once it activates (assuming you can't prevent that indefinitely due to the specifics of the engine being played). That would be a problem if the game were to last "forever". But the game doesn't last forever. You
do get to draw with TH, just not forever. Heck, you can look at it as a one shot +3 cards +1 buy that gives you a gold and still see value in that.
There is a difference between an engine wanting gold (for lack of better treasure) for $5, and an engine wanting a +3 cards +1 buy that turns into a gold for $5. The former is just an extra stop card that helps you buy things The latter helps you get a good turn relative to anything else you might be doing on that board before you get the gold. With that good turn, you can better tune your engine to make it more reliable or more explosive. You don't have to worry about payload so much anymore because TH covered that for you. If there is no other draw on the board, that +3 cards and +1 buy, even if temporary, is likely going to vault you ahead of an opponent that isn't increasing their hand size.
One trick is to just buy more THs. When you buy the second TH after drawing your deck, it's more like it costs $2 instead of $5 thanks to the gold it gains. The next one kinda costs -$1, because now you have two golds that together give $6. You can keep doing this, getting a further discount on future THs thanks to the golds gained by the last one. In the meantime, you are doing whatever else your engine does, eased by the albeit temporary draw of TH. When you finally decide to stop buying TH, well now you have a bunch of extra money with your golds that the THs gained you, plus whatever +buy you got from your last THs.
It's a matter of context. I would not say no to a magical Gold that was added to my hand as a sixth card every turn vs. nothing. Treasures like Gold are often problematic to for an engine because there might not be enough draw available on the board to draw the treasure and everything else that needs to be drawn. But the treasures gained by TH are always gained in the presence of something that can draw a bunch of cards: TH (until they run out).
The base case for TH is quite good, and you should get comfortable with understanding how it works out in games rather than always picking every other draw card over it.
Of course, TH isn't good everywhere. It's not good when you can only play one terminal each turn and you don't want that terminal to be a smithy with +buy for whatever reason. It's not good if you are going to need draw that lasts over many turns due to the particular nature of the board, for example one focusing on a repeatable source of VP tokens, or one that is using brutal attacks and both players are struggling to draw the cards they need to stay afloat.