I had an S&S with Charlie. I was 4cms dilated already with lots of consistent pre-labour contractions and the S&S actually made things go all wonky. I think had I not had one, I would probably have progressed at my own pace, but instead I lost quite a lot of blood and mucous, my contractions became irregular and, yes, it was not just uncomfortable, it was downright painful. I would never agree to another one should I find myself in the same position.
Yes, it can sometimes bring on labour, but often it doesn't. And
IMO it can mask the 'real' signs of impending labour, like the loss of your plug and regular contractions. And just to answer your last question, an S&S is like an internal, where your caregiver puts their fingers in and 'stretches' your cervix and 'sweeps' the membranes away from the cervix.