During the bleak times of no magic, spellwork took inordinate efforts to achieve the simplest of results. Since there are no books (or scrolls) to lead magic users in their craft in Fantastic America, trial and error has become the only way to learn the possibilities magic offers them. Accidents, some deadly, are bound to happen.
An important aspect of using magic is the toll it takes on spell casters. Regardless of the kind involved, mental stamina and sheer willpower control how magic operates. In the new magical age, even the most experienced witches lack the stamina to cast more than the most basic spells. Magical abilities take far less stamina, but also require practice to achieve proficiency.
The energies involved in magic change its users. Each kind of magic has a base emotion that makes that magic more potent. Rage, calm, cold reason, hope, passion, and cruelty, all improve the potency and durability of related spellwork. Contrariwise, the lack of the proper emotional state can harm the spell or decrease its effectiveness. Discovering this through trial and error will pose a steep learning curve as well.
These difficulties make magic tiring. Physically, magic leaves few obvious signs to outside observers. Mentally and emotionally, spellwork takes a heavy toll. Wizards, arcanists, and familiars must rest and relax after using their powers for extended times or major spell casting. The alternative can lead to loss of control, and permanent damage to their ability to use magic at all.
In the short term, magic will be difficult and dangerous to wield, its results will vary widely. Once magic users begin to master the aspect of magic they have access to, the dangers and unpredictability will decrease dramatically. There will always be a human element in spellwork prone to mistakes, but practice and survival will remove the inept from the world, like any other endeavor.