When you do the upper arm mod, and the car settles after you drive it, the spring perch will be in the same spot it was before vertically - it has to be since it is supporting the same load. But since the upper arm mount is now lower, the upper (and lower) ball joint will be HIGHER. And to do that they both swing in an arc, so they may be further out (or in) than before.
I am aware of the unibody "safety feature" (ie it causes a severe understeer or push) such that when you corner, the spring compresses and the upper ball joint pushes out, causing severe positive camber on the outer wheel and so lousy cornering.
Lowering the arm SHOULD mean the upper arm will now be out further than it used to be sitting static, so after you move the arms you will have positive camber and have to move the lower eccentric to fix this. Is there enough movement in the lower eccentric to do this? Either there is or there isn't.
If new upper arms come with the kit, the new upper arms might be a different length (ie shorter) to keep the lower camber eccentric nominally in its center position, allowing for plenty of adjustment.
If you KNOW this kit works and there is plenty of camber adjustment left then go for it, but I can see why a mechanic would not "guarantee" that everything will be ok.
Also, this mod will make the car want to oversteer (ie spin out) so some mechanics might not want to be associated with touching it so they don't get blamed for wrenching a car into a non-factory loose condition.
Not really an answer but an explanation of why they are hesitant perhaps.....
