This might be easy to misread, so just a clarification.
Taking -2 Sanity is not the same as being dealt horror, I.E this is not horror you suffer or a permanent reduction in horror health. Therefore the net benefit of St. Hubert's Key is:
In other words, this card gives you the exact same benefit as a Holy Rosary, and then throws in the extra +1. The difference is however that St. Hubert's Key self-destructs and automatically heals you when you hit your reduced Sanity threshold, thus while St. Hubert's Key gives you a bunch of extra power and safety you must still be careful or you'll loose it faster then you'dd like.
St. Hubert's Key does not invalidate Holy Rosary, the latter is still a much tougher asset than the former and in the mystic's already expensive lineup of key assets finding the extra cash to pay for an often unused boost is hard. Often the St. Hubert's Key will find itself sought after by non- characters like Daisy Walker, "Ashcan" Pete or Rex Murphy who will better appreciate the extra since they spend a lot of their time using that skill to begin with. Note that the reduced Sanity stat has an interesting interaction with Desperate cards (Like Reckless Assault).
Currently the lineup of character's will prioritize tests over tests, and often paying the extra cash will just be a waste, since you will spend a lot of your time utilizing a Rite of Seeking or the like. Note that your experience with this card will vary with difficulty, an of 4 for example (like Jim Culver or Father Mateo with the St. Hubert's Key) will get you much further against difficulty 2 locations in standard than it does in hard and it'll be much easier to bridge the gap for higher difficulty locations with other cards. An of 4 will succeed on a difficulty 2 investigate in The Gathering 80% of the time on standard but just 60% of the time in hard, that's the difference between finishing a 2 clue location (Study) in one round or two.
EDIT: Bonus note, give special consideration to St. Hubert's Key in solo, especially Solo on standard.