"There are other horrible things, but they are for the most part less dramatically unpleasant than being on fire. The main problem is that if you do catch fire, something went wrong, and it's very difficult to actually figure out what when you're the one casting the spell, and further mistakes will generally set you on fire again. Or encase you in a thin layer of rock, or generate a cloud of butterflies, or turn your fingernails into wasps. Hence the reliance on apprenticeship."