For those who don't have faith, or expect the Tarot to give debunk answers, it will. If a person believes the Tarot is not to be trusted, or it simply a parlor trick, the Tarot is so powerful that it will live up to that person's expectations to re-enforce those expectations. It will show examples of that person's current beliefs, and pictures. The Tarot mirrors the sitter's state of mind right here, in the right now.
On the other hand, if a person has an open mind to see the cards as an opportunity to expose patterns, and IF someone chooses to see opportunities the Tarot is revealing, and IF free will is used to make adjustments to outdated beliefs, etc, a person can have the opportunity to be blown away with the amazing power of the Tarot. Anyone can choose to "change your mind, and change your world" by adjusting to lessons within the cards.
There are many who believe that a deck of cards is not run by "spirits". Instead Tarot is simply a mirror of of your world, reflected back to you in symbolism. You have the choice to look at that symbolism and explore messages that your intuition helps you see, and decipher as a reader, or as a querent.
The Tarot show you examples of what your patterns and beliefs are right here, right now. It shows you what is going on right now, at the moment the cards are pulled. If you change your mind (and actions) right now, your cards will change to reflect that by reflecting your new state of 'now'.
ETA: There are many who believe, if a reading is not accurate, it is not the cards, but instead, the way the cards ended up being read. There is also a theory that the "Future" position, is actually "Probable Future" since there is always free will to change current beliefs, patterns, etc.
Also, since readers are human, there is no such thing as 100% perfect...