I have never used Cosco because I don't have membership and I don't believe in membership store. I have used Walgreens, CVS, Target, Meijer, Walmart and Ritz/Wolf processing. The results are mixed, I can't say anyone is better than another. They all do exceptable jobs at on time and terrible job another time. As for equipment, they all have good equipment and most of them a also fairly new. Some stores like Target, I notice that they just updated their equipment about 2 years ago. Walgreens used to have the Gretag equipment but they since upgraded to either Noritsu or Fuji. The equipment is not to blame except if you want optically printed prints then they don't have it. All of their equipment makes prints from a scan.
If you bring them JPEG files, all of them do a decent job that proves that their printers are in good calibration. When they print from a negative, the color balance and density depend either on the judgment of the operator or the machine automatic white balance and exposure. The results are mixed, some are OK some are bad. In this respect they are actually doing a better job nowaday than in the past since all of their equipment now allows the operator to preview the results on the monitor and make adjustment accordingly. Back in the 80's, most operators had to make judgment looking at the negatives only.
For processing of the negative, namely the C41 process, their quality is way down. Years ago most of the 1hr. photo lab had from 50 to over 100 rolls of 35mm film to process a day. In the morning before the store opens, the guy in charge (I was one of them guy back in the early 80's) would run the control strip, plot the chart and see if any adjustment needed to be made to the process. With large volume and a daily monitoring of the process, most stores back then had their process very stable and in very good control. Nowaday, the volume went way down and with low volume it's very difficult to keep the process stable and with the cost of the relatively expensive test strip to run only to process a few rolls I doubt that most stores nowaday run test strip daily any more. I have found the horible situation where they would shut down the C41 processor and only turn it on when they have film to run. Doing this the temperture of the process is not stable as the temperture controller has to hunt for the right temperture a while to get the temperture stable. With low volume it's very difficult to set a replenishment rate correctly to keep the process stable. They don't seem to clean their machine often enough (may be never?) so that there are a lot of lump and junk sticking to the negative and also scratches the negative. Their scanners also seems to scratch the negative near the edges of the image. The scratches generally won't show up in the prints that they printed because they do crop quite a bit.
As far as training, Wolf/Ritz has the people who do processing only do processing so I would expect them to be better trained but the results don't seem to confirm this. With Target and Walgreens just any employees can be made to do the photo processing stuff so I doubt that they can have much training.
I am now only concern with C41 processing, I don't care about the printing side any more because C41 processing at home is quite expensive and not easy to do it well. As for printing a few of the very high quality prints, I can print them in my darkroom or I can just scan the negative and print them on my own printer. No problem there. For high volume 4x6 printing (giving copies to other people) I would scan my negatives, doing color balance and density adjustment in PS and then save them as JPEG to give them to the stores for printing. This way I would take the operator judgment out of the process. Newly opened Walmart stores don't have the C41 processor any more and they use a new (don't know what brand) inkjet printer to do the printing. They do very good job printing from JPEG files and very economically. As for 120/220 negative, most stores do have the capability to process them but they simply don't want to do it.