Labs can screw up - of course, as nobody is infallible! It is however surprising how rare it is. I have rarely used labs for bl/w - but I have had films damaged with color stock. If you are shooting commercially, you do cover your a** by giving them the film in two batches - never entrusting the result of a shoot to one run.
Liability wise - they have to replace film and thats all. Makes sense as we all would claim that the fogged/black/scratched/blotched neg would have been a Pulitzer prize winning shot too!
Developing your own is convinient/fast and once you "wasted" some film on trials and errors - you got it made. You can also control the end result much better as you can correct for the light at the time of the shot by pulling/pushing, you can select a film stock tailored to the subject matter and you can fiddle with dilutions or developers to get exactly what you want. The Lab, by the nature of its business will have to develop for the majority of the rolls coming in - "averaging" the result.
Yes, you can screw up too - and probably will, but in the end it si better to do your own.
Over the years I have developed more than 30 000 rolls, but my own stuff and commercial stuff - and yes, I have made just about every mistake possible - but it doesn't happen often.
