You might have missed that the cards says "with a combined cost of UP TO 8" which is why there are no rule ambiguities (you could construct a hypertheoretical case in which the Curses and Coppers are out and the card becomes unclear but it is practically irrelevant; I never played a game of Dominion in which the Copper pile was even nearing depletion). So if the first card you gain cost 5 the second one can cost up to 3.
Ruins are obviously possible as both cost individually up to 5 and together up to 8.
I like the card, it is probably one of the strongest engine enablers. If the Kingdom allows for engine play there have to be good 4s respectively goods 5s and 2s/3s. In a money Kingdom it will probably suck but Duchy+Silver might be a thing.
No, I specified that I was talking about a situation when there are no cards costing
or less. Yes, it's super rare; but that just means that it's a rules ambiguity that will practically never happen, instead of a rules ambiguity that will rarely happen. It's still the exact same rules ambiguity.
The reason Ruins is ambiguous is because to gain 2 of them means you would have to first gain one card, then gain the second card. It is not clear from "gain 2 cards" if that's how it would work; as opposed to choosing 2 cards and gaining them both.
Edit before posting: Found this super old thread where I think Donald actually answers that question:
http://forum.dominionstrategy.com/index.php?topic=5990.msg161157#msg161157Basically, as with Embargo, "gain 2 cards" implies 2 separate gain events that should happen one after the other. So yes, you should indeed be able to gain 2 separate Ruins (or Knights, if you have cost reduction). But that still has the other uncertainty... do you have to consider your second gain's cost when choosing the first gain? Can you choose a
card for your first gain if everything in the Supply costs
or more? If not, you need to "look ahead" to actually make both choices together before gaining either one.