As others have stated, it depends on the staff. Where I am the local drug stores offering in house C-41 service are CVS and Rite-Aid, and Rite-Aid does send out E-6 (and other films like 120 but I don't use that service). They do a good job, and I have only found a mark on my print once, whatever it was I was able to just rewash the strip of film and Photo Flow it and it was fine.
The girls at Rite-Aid have done hundreds of rolls of film for me and that was the one time that I had a problem, that wasn't my fault, I once had them process a roll of film that I didn't expose so I can't blame them for that. I will admit that when I see new staff behind the photo counter i get a little scared, but they do fine.