The extra explorers are fun. When the enemy suddenly ses 3 elite units die befire even getting into fight. The extra explorers are only useful in discovery and industrial, and they aren't that much better than the explorer dog in discovery (nd dont say card value, you could save your discovery card until it becomes a industrial card.
Admirality is decent card if just for the extra monitor. 3 special monitor attacks can cripple a base. That is where non-port hot air-baloons come in, artillery spotters.
One card I find pretty much useless is portuguese hot-air baloons. Sure it gives you a second recon ability, but I find it redundant when you have spyglass, though with them becoming permanent in TWC, it may be good.
No card is "useless" but many cards are so badly outclassed that they have no place in any deck, except maybe peole who havent got better cards yet.
What about the cheaper first age market techs? I believe it is dutch, so they have villies, bank cards, or a decent explorer to send in discovery. THough maybe it isnt the Dutch who have it, as I can barely remember it.