The brakes on my 98 Olds Achieva always grab and slip really bad when they are damp. It feels like you aren’t going to stop. But after you put them on a few times, it’s like it gets the moisture off them and they are fine. My mechanic said GM cars are just like that, and there was nothing they could do, and they are safe. I don’t feel like they are safe. By the way, they do this whether new brake pads or not. They are not worn.
I bought my 87 Ranger new & on damp mornings it would do the same.
I simply rode/lightly applied the brakes for the 1st hundred feet of driving to dry them out.
Other than that the brakes were excellent & after I did the first brake job at 140,000 miles the problem went away.
Usually a change of brake pad material will correct the problem.
Sounds as if your tech is ill-informed or lazy. That is not typical. If it happens all the time you may have a leaking wheel cylinder.