Taxman is way worse than mine (and Mine is one of the worst 5-costs). It effectively decreases your handsize by 3 (!), so you better have a ton of draw. And if you have that much draw, the attack is probably irrelevant (opponent will just draw it all anyway), and so it effectively says "discard a treasure, if you do, gain a peddler" (assuming you reliably draw your deck). This means that it takes 4 turns to have payed out more than a silver. Do you have those four turns (and 4 actions and 4 cards)? It's just awful.
You don't reliably draw your deck, you're going for Taxman.
In the type of game where nothing's going on, any early hand with Taxman in it is better than the same hand with a Silver instead of Taxman. For instance, if you have a Silver and 4 Coppers, you can buy a Gold, but if you have a Taxman and 4 Coppers, you can turn one of those Coppers into a Silver and buy a Silver (which is roughly as good as buying a Gold) and you attack your opponent. If you have a Silver and 3 Coppers, you can buy a Silver, but if you have a Taxman and 3 Coppers, you get that same benefit while you're also trashing a Copper and attacking your opponent. It's only worse than Silver in your current hand when you're greening, and in that case, you can utilize the topdecking ability to set up big turns and make your opponent's turns significantly weaker.
If it's a kingdom where you want to buy something other than a Silver whenever you have $5, Taxman is not very good. Which is a lot of the time, but not every time, and the games where Taxman is useful are more common than the games where Scout is useful and the Taxman in those games is also more powerful than the Scout in those other games.
Taxman is pretty good actually, definitely not one of the bottom 5 $4 cards. It's a slightly weaker, but also cheaper Mine with a Cutpurse-ish attack to boot. It's strong on boards where Mine would also be strong - engine boards with good draw and +Buy, but no other Copper trashing. It's especially awesome in Colony games.
Engine boards with good draw and +buy but no Copper trashing are where Coppersmith is good. Taxman is incredibly weak engine payload, and if there's no trashing and no better payload, it's probably not an engine board. If there is better payload, you want that instead of Taxman.
Every card can be useful, but Scout is probably the card for which those situations are rarest.
I don't think that they are the rarest for Scout. Coppersmith, Thief, Harvest, Tribute, Adventurer and Transmute definitely might be rarer. The main problem is that even when Scout is actually useful, it's not gamebreaking, a central part of your strategy or even strong. It's just so very slightly better than nothing that it makes hardly any difference even when it's a good idea to buy it. That could also be said of Harvest and Adventurer (and I think they are pretty much as awful as Scout), but cards like Coppersmith and Thief are better than Scout because they have the potential to be incredibly strong at least once per 90-93 games where the card appears.
Coppersmith generally has 2 roles: early game accelerator and massive $ payload.
It's not really any better than Silver at the former.