A nice little hack to fullfil the limitation of the Chalice's second option is targeting a low sanity investigator and equipping St. Hubert's Key; it helps keep the Key in play while also being able to get rid of the doom easily. An investigator that can take those cards and benefit from the synergy is... "Ashcan" Pete! What's more, with his ability it's also possible to use the first option of Hallowed Chalice twice in rounds the agenda would advance anyway.

Not an insane combo, but you might get more out of this card than you think.

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I built a Dexter Drake that is a flex fighter, and this card seems invaluable. If you're taking Arcane Research (or two copies of it), In the Thick of It, and maybe even Charon's Obol, Hallowed Chalice looks amazing for survivability (especially if you can tutor 'Charm' with Molly Maxwell). Surprisingly, aside from Sword Cane and upgrading to Cyclopean Hammer late game, Dex's hand slot can be relatively uncontested if you're relying on Arcane slots to do your damage dealing.

Dex is actually the surprise winner of the Scarlet Keys mystic cards suite. Power Word, String of Curses, Uncage Soul (3) all seem pretty powerful with him, and even the Rogue cards such as Kicking the Hornet's Nest, Underworld Market, and Dirty Fighting can find a home with him.

The chalice is costly. Three bucks and it takes up a slot any investigator can’t have enough of. There are two people who this will be an excellent fit for, though. Amina, the doom manipulator, can play it for free then immediately undoom it by healing one damage or horror. Hopefully someone in your party is moderately undamaged. But this also a sometimes-pick for Dr. Carolyn Fern. Not because it’s an on-tap money generator for other players, Stand Together can do that more efficiently, and the Book of Psalms can add blesses at the same time. The reason why she'd like it: it’s one of the few efficient ways to heal damage at level zero. First aid, healing words, thermos: just as expensive, but with limited uses. if you have some wiggle in the encounter deck, you know you’ll have a way to heal someone’s horror completely and use the chalice to top it off. With hypnotic therapy, you only have to get within two horror of Sane.

Drink up!

So this card is actually not bad in true solo. It is expensive, yes, but the unlimited heals on it can really be helpful at times. You can always make use of the doom heal during witching hour turns. Agnes can get some good use out of this, but so can any low health mystic.

There are some neat synergies with the new cards in TSK as well. Amina can use Word of Woe to get herself some extra healing without using an action (and also possibly clear the doom by healing the last point with her next action); Sin-Eater can also get extra healing juice as well, if you really need it (and in true solo, sometimes you do).

Amina can also pay for it easily too, either with her ability, or even with Sleuth, which may see new life in Amina decks. The chalice is also not a bad pick to slot in with the new Astral Mirror.

Obviously, one must be wary of the agenda clock when using the chalice, but that's the cost for dealing with doom!

tl;dr, Vinnie, Carolyn, possibly Amina and Marie. Not a complete sink in Dr. Orpheus who can load up the doom and sell it. Extremely situational otherwise.

An important thing to keep in mind when playing this is that the only damage/horror that matters is the last one. Damage and horror is just a clock you're working against; turning back that clock is good, but not if you're wasting time on a different clock. Spending time healing when you don't need to is counterproductive.

This is $3 to play, which is a lot, and it occupies a very valuable slot. Both of those resources could be contributing to you actually making progress in a scenario.

One action to heal one damage/horror is already a pretty bad exchange outside of the direst circumstances. Three dollars (each worth slightly less than an action), plus one action to equip it, plus the opportunity cost of giving up a hand slot is abysmal. You never want to use the second action on it's own unless you're forced to, and if you are forced to, it's probably because you didn't play a better card in your hand slot.

Now, healing two for one action is better. But to do that, you're adding a doom. Assuming there are no scenario effects that care about doom (which isn't a guarantee), doom only matters when it will push the agenda. The problem is that healing exists to fix a problem, and you have no guarantee that that problem will align with the window this gives you to solve it.

Most of the time, when someone has taken three damage, it doesn't matter. That's a perfectly safe amount of damage to leave sitting on your investigator. You can easily finish a scenario coasting on 3+ remaining health, and you want to, because that means you didn't waste time healing when you could have been investigating. But because you have to clear the last damage to remove the doom, you need to use the chalice right now, before you know if it will matter. It also means you're following around the injured investigator regardless of what you each need to do to make progress, because it requires two actions across two separate turns. And if you're one doom away from advancing the agenda (or two doom if the ubiquitous Ancient Evils and its ilk are in play), you can't use it now when you need to without potentially sabotaging everything else.

The one time the first ability is totally safe to use is the turn you know the agenda will advance (which again thanks Ancient Evils isn't guaranteed), but unlike, say Blood Pact, the chalice exhausts itself, so you only get a single use within that often once-per-game window, and that's assuming you don't have more important things to do (which you always do).

And sure, abilities like Moonlight Ritual exist, but if you're using that to clear doom from the chalice, you're now spending $3, two cards, and a minimum of three actions to solve a problem that could have been prevented by any cheap soak in the game, which usually has other utility beyond healing.

In summary, it's a trap, and only useful for certain investigators that can piggyback some other, better effect on top of the healing.