It's all down to your local branch imo. Mine is absolutely lovely, and it's mostly down to the chair who is a fantastic, genuinely kind and wonderful human being. (She quite literally probably saved my life when I was home alone w a 2 yr old while my husband was out of the country, I had a blood clot, the GP wouldn't believe me and I needed child care to be seen at hospital. I have no family here and at the time, no friends. In fact, I barely even knew her at the time, she just rang me at the right moment for some random reason as I was sitting on the floor trying not to move much because I was terrified of something happening!) She now is a paid NCT employee which is brilliant because she deserves it.
Anyway I digress. The tone of these groups, as with any, comes from the top down and if you have a chair who is competing for the invisible prize in "least amount of pain meds during birth" or whatever, people will engage in that, and the only volunteers you'll keep are those who care about that sort of thing. We did have a few when I first started volunteering, but I (and others) called them out on it when it happened (I had an epidural and I'll absolutely respond when someone says "oh dear could you just not take the pain then?" Similar exchanges would happen between hardcore BFers vs those who chose not to BF with second children, or whatever reason which wasn't anyone elses business!)
I'm using those as examples because they're typical baby discussions. Basically though, it was/is a nice inclusive group of mums. Our biggest fault is probably that we were all about 30 or over when we had our babies, so the average age skews higher than the Sure Start groups did. Which will obviously have an effect on where people are with careers etc. I don't know.
I still help out with the Nearly New Sales though, and my son is 10.
(oh fwiw, I never did any of the NCT classes, and I'm glad I didn't. My prenatal group was lovely and half the cost of the NCT one)