My understanding is the guys behind the counter don't do a whole ton for digital prints. The machines are probably pretty self-diagnosing and nearly idiot proof. I would also guess that different locations use very similar or identical equipment.
My experience for cheap 1hr prints puts Costco ahead of others in price/performance.
There's a certain amount of maintenance involved, and there are some machine adjustments as well. The machines can certainly produce prints with iffy color accuracy and crooked cropping if not maintained and adjusted conscientiously. The equipment used by Costco varies about as much as the equipment used by the photo printing industry in general. My old location had multiple different models of printer in-house, and this is not abnormal. If the machines were as fool proof as you suggest, it wouldn't be so easy to go out and get an absolutely terrible print made. It's not a manual process, but there's still ample opportunity to apply destructive settings, and some of the defaults are not ideal.
Honestly, I think we're seeing this the same way from different perspectives. Is costco almost certainly better than places like Walgreens, target, etc.? Absolutely. Will it compete with that last little bit that the better dedicated photo-printing places can offer? That's likely going to depend on the specific store.
I once got an 11x14 from Costco that wasn't cropped straight, with a little bit of white border showing. WHCC calls me
if I do something like submit a poor-quality source file or have a border oriented oddly, to check if I want to resend files to correct the situation. MPix will have someone color-correct your photos by hand, if you desire. If I want high color accuracy at Costco, I need to tell them what printer to print on, and prepare my images accordingly. I'm not sure to what standard their other printers are maintained, or even what standard is kept for the profiled printer between profilings.
It really comes down to just how high of a standard you are setting and in what areas. I fully expect that costco will generally use good materials, have their machines adjusted sensibly, and normally maintain them decently. At a minimum, I would expect adequate prints, which is more that I can say about some other places. The last little bit to get to excellency is likely to vary by location.