I think there are a variety of ways to measure competence, experience being only one variable, but not everyone agrees on their validity. And even with basic competence, effectiveness will always vary client to client. The same is true for teachers: basic competencies can be measured, but not every competent teacher is the most effective teacher for every student. I just prefer to stack the odds in my favor by looking at a wide variety of criteria, rather than say it's all a gamble and take your chances.