I don't know if this is necessarily due to being a "big" lab, but it is more frequent with big labs due to volume and target customer:
2. Poor overall development due to exhausted/contaminated chemistry
1. My typical experience with big labs (i.e., drop off the roll at Walgreens to be "sent out") is scratched negatives. Of the last several rolls developed this way, there were scratches to some extent on every one of them. Often, its one or more scratches that extend allll. the. way. from one end to the other. I think this is from volume, and employees who are not necessarily photographically oriented people, and the distance between the customer and the folks running the film through the machine. You have to clean the machines often to get the scratch-making stuff out. You have to check frequently to see when those occur. You have to treat films appropriately (avoid contamination, etc.).
2. Sometimes, not always, the negatives come back really dark, or with obvious poor development. This is usually because of volume. Need to replace (not just replenish) chemistry more often with volume. Gunk builds up quickly with volume. Chems get exhausted quickly with volume. Need to check frequently to see when cleaning and replacement is needed. Employees who are not invested in the happiness of professional customers and not going to do as thorough a cleaning of the machines. Think of those teenagers at the fast food restaurant -- not likely they're going to deep clean the grill until it shines like a diamond, eh (one reason I don't eat at such places)
3. Lost film. Again, due to volume. I've had film lost forever by "big" labs. I can only imagine the hundreds (thousands) of packets those labs process every day. Incoming, outgoing, record keeping, fast-paced environment (I imagine). I can see how the occasional roll/negatives/prints gets lost. With smaller labs, the less volume should decrease this possibility. But, can still happen...
I develop at home. B+W only now. I do have a fair amount of C-41 in the fridge/freezer though. I might burn through some on low-priority jobs/subjects and .....you guessed it.....send them to the big lab.