I think the distinction is that "+1 Buy" enables you to get some benefit out of having the card in your deck, however niche a situation it might be when you want it. But "+1 Card" (combos with specific other cards aside) only ever gives you what you would have had if you hadn't had it in the first place, so it doesn't really help.
+1 Card - Eats your action.
+1 Action - Eats a card slot.
+1 Buy - Gives you +Buy! Now I can get two Provinces!
+$1 - Gives you a coin! Now I can afford one Province!
Now, nothing says that all the Ruins cards have to be potentially useful, nor that they need to be roughly equivalent in strength. But there IS a fundamental difference between the first two, which require combos to do anything for you, and the last two, which are simply meager benefits.
I'm not sure what you're saying here. All of them but the +1 Action one "eat" your action. And all but the + Card one "eat a card slot. All of them need some sort of combo to actually be useful. I don't see why +$1 is considered a "meager benefit" and +1 Card isn't. +$1 might be better than +1 Card most of the time, since it won't draw your other ruinses dead, but it's not a ton better, though I guess you might also buy it in a Minion or other draw-up-to-X deck with enough villages. But it's not fundamentally a whole lot different.
Ok, if you have a card that has "+1 Card" on it, then playing it will draw
the card you would have had anyway if you hadn't had that that Ruins in your deck. Thus, in the absence of specific combos (e.g., KC), the only real effect is to take something away from you: your Action, specifically, or your card slot should you refrain from playing it at all.
But a card saying "+1 Buy" gives you something you might not have been able to have otherwise. It takes away a card slot and your action, but you get a buy that you would not have had had you not had the Ruins in your deck. It may or may not be a trade-off you would welcome, but it
is a trade-off, as opposed to purely a negative.