If you count digging attacks, there's also Saboteur.
Digging-for-X is a tricky mechanic. I don't think there's anything fundamentally wrong with it, and I don't think the list of Diggers is any more or less balanced than your average random 10 cards (aside from Rebuild being an outlier). However, I do think that designing digging cards takes extra design considerations, because there are some potential traps to it.
The thing with Diggers is, you don't get to pick which card is actually Dug. With a KC or a Throne Room you can choose any Action you have in hand, and you know exactly what it's hitting when you play it (edge case: chained TR/KC + draw cards). By contrast, with Golem, you don't know exactly what action it's going to hit in general. You don't get to pick that turn. They way you choose which action to play with Golem, is by being extra selective about what actions you acquire in the rest of the game.
And that's really the big issue surrounding Dig-for-X: They exert a disproportionate influence on the composition of the rest of your deck. Hunting Party is monotonous, because you can't just splash in a minor utility card because that makes HP choke. Golem is fine, but you have to be careful and not let the riff-raff in. Venture and Adventurer are both basically crap unless you can clear out your Copper. With other cards you have different ways of dealing with this. Like the aforementioned TR/KC, you can have some mediocre actions lying around and use sifting or draw to make sure you KC the action you want. Golem, there's no way to deal except exclusivity.
Personally, I don't there's a problem with how the digging mechanic works. I like the change-up in how you have to strategize your deck, and plan ahead to a larger extent than normal. If they were more common I might start feeling constrained, but as-is I don't have an issue. Hunting Party is the only card where I feel that Digging causes a problem, because it wants your deck to be monotonous. Golem, I think the card's issue is cost, and Digging is completely unrelated to it being underused. Rebuild, Digging ups the power but I don't think that it's central to the card's problem. Sage I think is under-rated, and Adventurer is situational but not bad.