- You can only have 1 copy of your investigator's signature card in your deck. These cards have no level, which is different from having level 0, and the deckbuilding rules only allow you to add cards of level 0-5 to your main deck. (Note: Signature cards in the Core Set were accidentally misprinted using the same template as level 0 cards. Starting with The Dunwich Legacy deluxe, signature cards will be printed as cards with no level (e.g. Jim's Trumpet).
Think of it this way: you have to spend an action to put the Heirloom into play. It costs three resources. And when you play it, you're giving up a card. How many cards does it have to draw to start before it generates real value above and beyond that 3-resource, 1-action, 1-card investment? I would say about six. Are you really going to play six spells over the course of the game? Even as Agnes, I would say that's a tall order. Bear in mind that several Agnes staples like Drawn to the Flame and Forbidden Knowledge do NOT have the spell keyword.
Moreover, playing the Heirloom will massively slow you down in the early game. Agnes has more strong assets she needs to get into play early than any other investigator. There's Shrivelling, of course, and Agnes really wants Peter Sylvestre as well. She probably needs Rite of Seeking or Flashlight to do some investigation, and lots of Agnes builds use further things like Fire Axe and Ritual Candles, among others. Can she really afford to invest critical early-game time and resources into a card that might, if everything breaks right, turn a very small profit over the course of the game? (But more probably will end up in the red?)
The Heirloom's effect only gets worse if it's drawn later in the game. If it's not played in the first couple turns, it has basically zero chance of paying back its cost.
It also eats up an Accessory slot, which is inconvenient because Holy Rosary is typically more powerful for Agnes.
Of course, the Heirloom is by no means a crippling thing for Agnes to have in her deck. She just uses it for +2 to a Strength or Will test. But all the other investigator-specific cards are at least occasionally worth actually playing. (Daisy's is a little borderline, but you can imagine scenarios where it would be worthwhile.) Agnes's is basically only skill test fodder.
With so much time having gone by since the release of this card, I thought it might be time for a re-appraisal. While I still think it's fairly weak, as far as signature cards go, the card pool has expanded vastly since the Core Set, giving this card a total of 20 spells to work with (including her weakness). Let's take a look at how likely it is to play six spells over the course of a game nowadays.
Very beneficial to Agnes is the fact that many newer spell events are fast. For instance, Aggy gets a free draw from Hypnotic Gaze, Ward of Protection, Mind Wipe, Counterspell and Time Warp. All of these are perfectly reasonable includes for Agnes, so her odds of getting a double benefit from them are good. In the same vein, many spell events take actions that you might be taking anyway, giving you another double benefit situation with her necklace out. These include Astral Travel, Bind Monster, Blinding Light and Storm of Spirits. Uncage the Soul isn't a spell, but it does put spells into play very cheaply for Agnes, so in a way it triggers the amulet. Getting a free draw for playing spells even makes some sub-par includes like Alchemical Transmutation and Recharge more attractive, since she gets the draw even if the spell fails. Running a doom-heavy deck? Moonlight Ritual is a spell! Free draws!
With the variety of spells necessary for this to be decent in place, we come to the other problem with the card: that darn accessory slot. While Holy Rosary is obviously terrific for Agnes, she has other options to bring her up to six at level 0; specifically David Renfield. I'm not saying she shouldn't run the Rosary; it's incredible on her. But I do think the Heirloom has enough triggers in the game now that it is competitive for the slot, and makes Relic Hunter a very tempting include for her.
Are all the problems solved? Is it worth playing late? Is it as good as Zoey's Cross? Absolutely not, but I think it's a much harder decision now whether to mulligan it out of the starting hand or play it first turn over Rosary. In a deck that's built around having spells to play, which is totally realistic now, it would be a very powerful card.
Spells continue to be released, and they can only make this more powerful.