IF your techy is certified with Bernina, s/he will be trained and know what they are doing.
IF you are dealing with someone that is a sewing machine service person, but not certified via Bernina, then .... find someone who is.
You seem to be flipping around from one dealer to the next.
You would be wise to stick with one ... and work with them, instead of thinking they are no good.
Earlier you mentioned that you are not as knowledgeable as you could be, so arrange with your dealer for some lesson(s). They are well worth it. Very often, it is not a machine issue, or a service/dealer issue ... and yes, the finger comes back to be operator error.
Yes, there is a LOT to learn, and sticking with one dealer/techy, you can develop a relationship, where each gets to know the other, and can work thru challenges you are facing together.