I will assume you are referring to spells. Spells are a 'push' to help something happen that could have happened without a spell. They need to be based in reality. You also have to go after what you want and not just rely on a spell.
example, if you do a spell to help you gain employment and then don't look for work, its not going to work
You cant get an unrealistic result either. If you are a waiter no spell will get you a job as a doctor.
No spell can give you an unrealistic outcome, no mermaids, werewolves, vampires, cartoon characters, make believe worlds or realms.
Hope this helps, I would strongly suggest you read a couple of good books on the basics of witchcraft so you get a better understanding.