Getting good prints from transparencies was always painful as far as I remember. Looking at the results has always disappointed me, because the original slides are so much better. Maybe it would have been easier not to look at the prints at all and give them away in a closed envelope - the majority of people that are used to get the crappy prints from big labs wouldn't have noticed anyway how bad they are.
The worst thing that once happened was that the lab lost all originals (as well as the prints) from the selected best shots of my sister's wedding. This was the moment in life I never gave any film or slide into a mass users lab again and turned to professional labs that do the job locally.
Unfortunately the pro labs are not all better. Especially with 120 film I have never experienced that the slides came back untouched. Sometimes finger prints, sometimes scratches, sometimes dirt. Even when they wear gloves, I would be interested to understand their washing cycles for the gloves, because sweaty gloves will cause smear on the slides as well. They are simply not able to handle 6x6 slides appropriately with an edge of 2mm on each side. I always use tweezers to keep them fat-free.
As prints are always secondary usage for me, because slide projection is intended, I most recently bought an Epson film scanner and I am happy to say that the labs can now scratch their displays, make as many finger prints on their monitors, and loose as many of my files as they want!