The more complex the spell and the more drastic the effect means you need to keep the spell matrix and math firmly in your mind and then you need enough magic to power it. This is part of the reason not many unicorns learn more than the basic spells and most go to magic schools to learn anything more. And also why only unicorns use this type of magic. If a earth pony or a pegasus did try this method they wouldn't be able to push the spell out of their bodies where it's been formed.