More surprises for me:
Plan, Smugglers, and Doctor so low.
Expedition and Scheme so high.
Plan: This is baffling for me, especially since Bonfire is so much higher. When building an engine, you often want 5+ copies of at least one card - preferably ASAP. Getting a trashing effect whenever you buy a copy is great! You can count on getting rid of all of your starting Estates and 2+ Coppers with an early plan. Plan is at its best when you can open with it on a 3/4 with a great 4-cost card (like Tournament, Ironmonger, Advisor, Herald, Conspirator, or the Village variants). It's much better than Bonfire in this case (-2 Coppers vs. -1 Estate with additional trashing just a buy away). Anti-synergizes with gainers, though.
Smugglers: In an engine mirror, this card is just too good to pass up. Gaining 5-6 cost cards from Smugglers and purchasing another piece during your buy phase is super powerful. If your opponent tries to play around it, then it works a little like an attack. This is one of my favorite gainers; I might take it over (or in addition to) Ironworks.
Doctor: The on-play effect is pretty strong (especially with Spy-effects). I believe it's the only card that can trash more than 1 card from outside of the hand/play. The overbuy effect can be very powerful, especially with a 5/2 opening, or an early 6-7 coins on turn 3.
Expedition: Is countered HARD by handsize attacks. Otherwise, decent lategame if you have an unlucky turn and there are no good 3- cost cards. Situational and low-impact.
Scheme: A solid card, but low impact. Scheme rarely contributes substantially to my strategy, it just adds a little bit of reliability at the expense of deck cycling.
Addition: In Tunnel's defense, it is still 2VP for 3 coins. So, even ignoring the Gold-gaining (which is situationally good), it can be an important element in maximizing VP in the endgame. 11 coins is enough for 8 VP. 6 coins is enough for 4 VP. With 8 coins and two Provinces in the pile, you can opt for Duchy+Tunnel for 5 VP.