Spells honestly are but words, its you that puts the energy and power behind them. I personally like to write my own because they are tailor made for the situation that I need them for. Rituals however I will use from others, depending on my goals.
Also keep in mind the laws of physics and what is and is not possible in the physical world. Be practical and logical about your workings.
Most of the spells that dont work here are fantasy spells that say you can change yourself into a vampire, mermaid, fairy, werewolf or things like that. There are things that even magic can not do in this world.