If you ask about the probability of success in Visa stamping, there is no such thing as "Probability" or "chance". It is either "Yes" or "No". Either you get approval or Rejection. Those are the 2 possibilities you have. Yes, there are cases where people get 221g for H1B, PIMS verification which might take from a week to months. There was a thread in sajha few months ago where a girl was stuck in ktm for more than 3/4 months and i don't know if she got approved or rejected..It truly depends on your luck, job, your answers and on the consular interviewing you. Take it just as when you went to interview for your F1 visa. What did you have on mind ? you'll have the same feeling when you go to US Embassy., Yes you'll get scared because you're trying to go to somebody's country and you need permission..
Compared to India and other countries, H1B or F1 stamping is fairly easy in Nepal, that's all i know but there's no such thing as 100 % guarantee.

Regarding answering at POE, make sure you tell them YES you have worked using SSN, You can't lie those people, within few seconds of entering your information in their computer, they can get your history..

Good Luck.