magic is energy found in nature, that's why it can't go against nature. nothing is impossible, but you have a 99% failure rate. i'm the type of person who looks at things logically before resorting to the mystical, and logic dictates energy and chanting 'grow wings' won't physically make it so. astrally speaking i suppose, but physically no. the power of the mind cannot make people become a panther or something. the mind can trick the body into believing it's sick, or some sugar pill worked, but it's not really happening, it's all in the persons head.
if you look at the history of vampires, there's plenty of reasons for this belief; people, diseases, hearsay, superstitions. by saying some words on a full moon and drinking some blood, it won't turn you into a vampire, no matter how strongly you belief, it just can't. of course, your mind might believe it's real, you'll consume raw meat, avoid sunlight, and do everything in a more vampiric way. you're not a real vampire however, just sanguine. which i have nothing against someone living a lifestyle, just know you're human. it's like positive affirmations, think positively and you'll only see good, and vice versa. it's no different with a spell, if you want something bad enough, you'll believe it worked.
certain ones can work, like to clear up your skin, little changes can be done if you combine your own desire and effort to acomplish it, the spells a motivator, a placebo, might speed up things, but no spells works over night. a friend of mine has a daughter with curlie hair, well she hates it [i personally love it but i have straight hair so whatever] every day for a month she'd use a hair straightener on her hair repeating a little chant she made up. by the end of the month, her hair was straight. but, it's not permanent, after a 4-6 weeks it starts to recurl, so one week a month she does the hair straightening thing. [did that for a few months before she gave up] point is, things can be changed with a little magical help, but you are human, if a human can't shapeshift, or fly, or whatever else, magic cannot alter it, since magic is a part of nature, as we all are. some is true if you cast a spell on a rabbit to eat meat, won't work.