Let's give an example: someone accidentally touches the wrong thing with a soldering iron. The motherboard is damaged. The repairer is honest and decides they have to fix the phone at their expense, but the simplest repair, and in reality probably the only practical one, is to replace the motherboard. Your data are in a storage chip that's fixed to the motherboard, so when that is replaced you lose everything.
That, or something similar, could hapen to anyone. Not saying its likely, but why take the chance?
Basically I would never give a phone to someone else to fix without backing up anything of importance first, no matter who that person is.
Ironically the really big risk would be an official service centre! Those places routinely wipe every phone they fix, either because it's policy to do a full update of the software while they have the device or, in some countries, because it's policy to do this to avoid any possible lawsuits because the technician accessed data from the device they were repairing.