When you just don't have the skills to do a job you are at the mercy of the tech and just have to hope for the best.
From most of the post I have read on here the mobile tech were competent and saved the day in that case you are happy to pay.
On the other hand when you have the skill and no longer can fix things due to age (me)you can oversee the work even though the mechanic really does not want you to.
It doesn't take long to figure out if the tech doesn't know the job and you intervene by telling him how to do it or fire him.