Pros
It is an easy job to get as a Native English speaker. They pay minimum wage & give you chances to excel. They help you get your social security number for Spain.
Cons
They exploit young workers by attracting them to move or live in their Spanish cities, but then giving them unpredictable work schedules (despite requests and disclaimers). They foster a hostile work environment by making young teachers feel they are constantly under evaluation and can be fired at any moment. Where they get you is that once you've dedicated the probation period, you don't gain any more trust, benefits, better work schedule, or higher paid. Instead you gain more responsibilities, and the deal with the side effects of a rapid turn over rate.