Spoiler (har har) This card has done well by me.
First, as any Noise player will tell you, Corps hate things falling out of R&D and into Archives without a run happening. Even when it isn't an agenda it annoys them.
But this card doesn't fire under the runners control. That would be crazy. It only goes off when the corp scores. In most games where the corp wins, they score 4 2 point agendas (or 3 2 pointers and a 1 pointer). Discounting the last score (since, obviously, who cares about milling when the game is over), that leaves the game divided into the first part, where you'd click once, spend a cred and over the course of the game you'd get 3 cards into archives, the second part (after their first score), where you'd 'only' be buying 2 cards into archives, and so on. Presumably, the runner won't install this card if the corp is at 5 points, so the worst state for this card is to give you 1 card into archives for 1 click and 1 cred.
The Corp's tools to mitigate this random milling is pretty much just Jackson. If they have a rezzed Jackson when they score, or a click left to install him, they can fling any agenda that falls back into R&D. Since for most runners Jackson is KOS, this is a surprisingly uncommon condition. He just doesn't usually live on the table, the double draw is too powerful. Obviously, they can stagger their scoring (install advance advance a 4/2, for example, so that they don't end up scoring last click), but Leela players can vouch for how wrong footed a corp gets when they have to play around like this.
Most games, the Spoiled cards end up in the trash without being saved. Pulling numbers out of the air, lets say most corps have 49 cards, of which 11 are agendas. Some decks will have more, others less, but that's a fair baseline. Let's further say that Spoilers fires twice during the game (my experience is that 3 times is more likely, but at least twice covers a supermajority of games). So 11/49 of the time you throw an agenda in the trash. For easy math lets say 1 in 5. The odds of missing 1 in 5 twice are 16 in 25, call it 2/3 against. Every runner knows what a 2/3 feels like, due to the psi game mechanic.
Spoilers impact on score is a lot like "take a psi game that neither side pays for, if the runner wins they get an agenda". For 1 credit and 1 click. Its also important to remember that 'losing' this psi game just means 2 non agendas end up in trash, which obviously isn't bad either.
The influence isn't attractive out of faction, but Anarchs should slap a singleton in their decks. It'll lucksack you wins that you wouldn't get otherwise.